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COMPARATIVE PERFORMANCE ANALYSIS OF MOD 303 EVACUATED TUBE
AND FLAT PLATE SOLAR THERMAL COLLECTORS
by
Garrett Starks
A Thesis
Submitted in partial fulfillment
of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
September 2015
©2015 by Garrett Starks. All rights reserved.
ABSTRACT
Comparative Performance Analysis of Mod 303 Evacuated Tube and Flat Plate Solar
Thermal Collectors
Garrett Starks
The Comparative Solar Project, a pilot study installed on residential building Mod
303 at The Evergreen State College, delivers hot water and radiant heat to eight student
residents. The project consists of two solar thermal collectors of comparable production
capacities: an Apricus AP-30 evacuated tube collector and a Caleffi NAS10410 flat plate
collector. This research evaluates the performance of each collector to determine the
effectiveness of solar thermal hot water and radiant heat on campus. Temperature of the
heat transfer fluid inside the Apricus AP-30 was measured every ten minutes for one
year, from May, 2014 to May, 2015, while data collection began full time on the Caleffi
NAS10410 in September after the system became fully operational. These measurements
were analyzed to determine which collector provided the most hot water throughout the
year and how each functioned at the specific site. The analysis revealed that the Caleffi
flat plate collector was capable of reaching much higher temperatures than the Apricus
AP-30. In September, 2014, the Caleffi circulated hot water approximately 90% of the
time while the Apricus circulated hot water for only 43% of the recorded hours. The
Apricus outperformed the Caleffi only during November and December, when low
temperatures and limited solar radiation severely affected heat production. System
maintenance took place during January and February, 2015, and no data collection
occurred until March. Throughout the data collection period, the flat plate collector
suffered several issues, such as stagnation, faulty sensor readings and system component
failure, all of which influenced the certainty of the results. A lack of available data
limited any further analysis and led to the conclusion that a comprehensive monitoring
system must be installed in order to fully understand how solar thermal hot water can
help The Evergreen State College achieve its carbon neutrality goals.
Table of Contents
LIST OF FIGURES ........................................................................................................... vi
LIST OF TABLES ............................................................................................................ vii
ACKNOWLEDGEMENTS ............................................................................................. viii
INTRODUCTION ...............................................................................................................1
Carbon Neutrality by 2020..............................................................................................1
History of the Comparative Solar Project .......................................................................4
SOLAR THERMAL TECHNOLOGY ................................................................................7
Low-Temperature Collectors ..........................................................................................7
Medium-Temperature Collectors ....................................................................................8
High-Temperature Collectors .......................................................................................10
Passive Systems ............................................................................................................12
Thermosyphon ..............................................................................................................13
Integral Collector Storage .............................................................................................14
Active Systems..............................................................................................................14
Direct or “Open Loop” Systems ...................................................................................15
Indirect or “Closed Loop” Systems ..............................................................................15
iv
Drainback Systems........................................................................................................15
METHODS ........................................................................................................................19
Data Sources .................................................................................................................19
Data Preparation............................................................................................................20
Identifying Research Limitations ..................................................................................21
Temperature Analysis ...................................................................................................24
RESULTS ..........................................................................................................................26
Collector Temperature Analysis ...................................................................................26
Emergency Shutoff Analysis ........................................................................................41
Collector Performance Summary ..................................................................................45
Validity of the Results ..................................................................................................46
DISCUSSION ....................................................................................................................47
Did the Comparative Solar Project Meet the Specified Goals ......................................47
Theoretical Savings and Carbon Offset Analysis .........................................................48
Moving Forward with Solar Thermal Hot Water Heating at Evergreen ......................50
CONCLUSION ..................................................................................................................53
Bibliography .................................................................................................................55
v
List of Figures
Figure 1: The Evergreen State College GHG Emissions (Mitigation and Offset Targets) .2
Figure 2: The Evergreen State College Greenhouse Gas Emissions Inventory...................3
Figure 3: Flat Plate Collector ...............................................................................................9
Figure 4: Evacuated Tube Collector ..................................................................................10
Figure 5: Concentrating Solar Collector ............................................................................11
Figure 6: Solar Hot Water System-Types Compared ........................................................12
Figure 7: Thermosyphon System .......................................................................................13
Figure 8: Integral Collector Storage ..................................................................................14
Figure 9: Active, Closed Loop Solar Water Heater ...........................................................16
Figure 10: Average Monthly Collector Temperatures .......................................................28
Figure 11: May 2014 Average Daily Collector Temperature and Solar Radiation ...........30
Figure 12: June 2014 Average Daily Collector Temperature and Solar Radiation ...........31
Figure 13: July 2014 Average Daily Collector Temperature and Solar Radiation ............32
Figure 14: August 2014 Average Daily Collector Temperature and Solar Radiation .......33
Figure 15: September 2014 Average Daily Collector Temperature and Solar Radiation .34
Figure 16: October 2014 Average Daily Collector Temperature and Solar Radiation......35
Figure 17: November 2014 Average Daily Collector Temperature and Solar Radiation..36
Figure 18: December 2014 Average Daily Collector Temperature and Solar Radiation ..37
Figure 19: March 2015 Average Daily Collector Temperature and Solar Radiation ........38
Figure 20: April 2015 Average Daily Collector Temperature and Solar Radiation ..........39
Figure 21: May 2015 Average Daily Collector Temperature and Solar Radiation ...........40
Figure 22: Flat Plate Temperatures ≥ 270° F .....................................................................43
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List of Tables
Table 1: Total Recorded Hours vs. Active Recorded Hours .............................................27
Table 2: Average Collector Temperatures and Solar Radiation per Month ......................28
Table 3: Total Recorded Hours vs. Active Recorded Hours .............................................44
vii
Acknowledgements
First and foremost, I would like to extend my sincerest gratitude to my reader,
Kathleen Saul, for her endless dedication, encouragement, and patience. Your mentorship
throughout this process has been the driving force behind my ability to complete this
thesis. I cannot express how much I appreciate your commitment to helping me overcome
all of the challenging situations I found myself facing throughout the past several years. I
couldn’t be more thankful for your support.
I would also like to thank Judy Cushing for all of the guidance she has given me
and knowledge she has shared over the past several years. Thank you for always making
it a priority to help me through my early thesis obstacles and continuing to extend your
support. I owe Martha Henderson a huge thank you for helping me with the early
selection and development of this thesis. Your advice helped me find direction and stay
focused on my goals.
I directed many inquiries about the solar thermal systems to Joe Martino and
Joseph Clevenger from the RAD Sustainability Crew. Thank you both for allowing me
access to your resources and hunting down answers to my questions. I truly appreciate
your contributions to this research. Thank you to Scott Morgan for introducing me to the
Comparative Solar Project and chasing down my random requests for information and
contacts. Thank you to Chelsea Waddell for all of the assistance with Excel and raw data
management. You saved me so much time and frustration.
Special thanks goes to Gail Wootan for her dedication to the MES program. You
have helped me so much during my time at Evergreen and put up with far too many
frantic emails and office visits. I am very appreciative of your efforts and your amazing
contributions to this program.
I am so grateful for all of my family and friends who endured countless hours of
listening to me discuss this thesis and encouraged me to push forward. I would like to
thank you all for not losing faith in me.
Finally, thank you to my parents for providing me with this wonderful
opportunity. You have made so many sacrifices that have allowed me to explore my
passions and ultimately pursue this degree. I am very lucky to have been able to learn and
grow under your guidance and I would like you to know that I have the utmost respect
and appreciation for you both. Thank you for everything.
viii
INTRODUCTION
Carbon Neutrality by 2020
In 2007, Thomas L. Purce, then President of The Evergreen State College, signed
the American College and University Presidents Climate Commitment (ACUPCC),
which set in motion the campus-wide pursuit of carbon neutrality. The ACUPCC strives
to unite higher education institutions with the common goal of “exercising leadership by
modeling ways to minimize global warming emissions, and by providing the knowledge
and the educated graduates to achieve climate neutrality”. In 2009, as part of this
commitment, The Evergreen State College published a climate action plan: Carbon
Neutrality by 2020. This plan was submitted to the ACUPCC as “an articulation of the
strategies, tactics, and resources required to achieve carbon neutrality by 2020”.
The Evergreen Carbon Neutrality report laid out a detailed framework with
“annual targets for progress in specific GHG categories and spells out specific mitigation
strategies within those categories”. The criteria and strategies for reducing greenhouse
gas emissions include:
●
●
●
●
●
●
Any action must be consistent with the mission and values of the college,
It should demonstrate financial efficiencies,
It should have a reasonable ease of implementation,
It should be achievable,
It should advance social, ecological, and economic sustainability,
Our plan should demonstrate flexibility and resilience to future changes.
Recommended strategies:
● Energy efficiency and conservation,
● On-site renewable energy production,
● Commuting efficiencies and transportation alternatives,
● Waste stream management, including purchasing and food management
processes, and
● Building and grounds infrastructure and practices.
1
Strategic approaches:
● Technical innovations,
● Increasing individual mindfulness and engagement with carbon neutral habits, and
● Institutional policy and procedural changes.
As part of the ACUPCC, The Evergreen State College develops annual
greenhouse gas emissions inventories. An original projection of greenhouse gas
emissions and targets from 2005-2020 was included in the climate action plan (see Figure
1). The data for 2005-2008 are actual values while the numbers for years following that
are projections.
Figure 1: The Evergreen State College GHG Emissions (Mitigation and Offset Targets).
2005-2008 are actual values. 2009-2020 are estimated projections. Carbon Neutrality by
2020. The Evergreen State College, 2009. Print.
As evident in Figure 1, electricity, space heating and commuting contributed to a
significant amount of Evergreen’s greenhouse gas emissions in 2005, approximately 69%
2
(Carbon Neutrality by 2020 18). Reducing emissions in these three categories became a
priority. Figure 2 below depicts the actual greenhouse gas emissions for 2005-2013. This
figure shows a dramatic decrease in electricity use while space heating and commuting
continued to contribute to greenhouse gas emissions. Renewable energy certificates
purchased by the college are responsible for offsetting greenhouse gas emissions from
electricity.
Figure 2: The Evergreen State College Greenhouse Gas Emissions Inventory. "Greenhouse Gas
Inventories." The Evergreen State College. The Evergreen State College, 2015. Web. 2 July
2015.
Since 2006, Evergreen has been able to claim that 100% of purchased electricity
used on campus comes from clean, renewable sources. In 2005, students approved a
Clean Energy Initiative to pay $1.00 per credit to pursue campus-wide renewable energy
efforts; ninety percent of this fee goes toward buying renewable energy certificates from
3
Puget Sound Energy and Tacoma Public Utilities to offset use of carbon based energy
sources (Clean Energy Initiative).The first renewable energy certificates were purchased
in October, 2005, three and a half months into the 2006 fiscal year. As stated in the
climate action plan, this is not a permanent solution for greenhouse gas emissions and
Evergreen will continue to explore sustainable options and make realistic institutional
changes to reduce its carbon footprint. Note that the other 10% of the fee funds oncampus renewable energy projects, such as the Comparative Solar Project, described
below.
History of the Comparative Solar Project
The Comparative Solar Project is a solar thermal hot water heater pilot study at
The Evergreen State College. The Comparative Solar Project initially received funding
through a $15,000 grant awarded in the 2010-2011 year by the Clean Energy Committee.
Project costs were estimated at $5,000 for equipment and $2,500 for maintenance per
collector, totaling $15,000. Established to test the performance and viability of two
different types of solar thermal collectors, the project provides hot water and space
heating to residential housing unit known as Modular Housing Unit 303 or Mod 303. The
grant application listed the specific goals of the Comparative Solar Project as:
● To install two different kinds of solar panels with similar collection capacities for
the purpose of comparison,
● To develop relationships between Residential and Dining Sustainability Crew
student-employees, faculty, and local companies involved in sustainable design,
and
● To foster a conversation between the above mentioned groups in order to develop
an academic understanding of the possibilities presented by solar hydronic
systems in the Pacific Northwest.
4
The Comparative Solar Project was designed following The Natural Step’s
Framework for Strategic Sustainable Development, built around four system conditions
for creating a sustainable society. The conditions state that:
nature is not subject to systematically increasing concentrations of substances
extracted from the Earth’s crust and produced by society, degradation by physical
means and people are not subject to conditions that systematically undermine their
capacity to meet their needs” (The Four System Conditions of a Sustainable Society).
The authors of the grant believed that “The Evergreen State College is in violation
of the first three requirements due to the methods used to power housing” (Solar
Handbook 2). In order to meet all four requirements, Evergreen needed to investigate the
many options for renewable and sustainable energy. The Comparative Solar Project
aimed to provide a working example of the potential of renewable energy infrastructure
in line with the College’s continuing efforts to become increasingly sustainable and
carbon free. In the 2011-2012 academic year, another grant application was submitted to
the Clean Energy Committee requesting $4,000 in funding to install a data collection and
monitoring system at Mod 303. The monitoring system would collect a wide variety of
data, including values for variables such as total energy generated, pumping energy, tank
temperature and system status (Solar Thermal Monitoring System 6). These variables
could then be used by students for a variety of analyses that would allow them to develop
“a basic understanding of the relationship between weather, climate, and renewable
energy generation; to more advanced multivariate analyses when including additional
data sources such as solar radiation or information from other campus renewable energy
installations or the campus meteorological station” (Solar Thermal Monitoring System 6).
Unfortunately, no funds were awarded and system data collection remained
limited to collector temperature only. The lack of a comprehensive monitoring system
5
severely hinders the ability to gain a deep understanding of the value of the system and
learning how this technology could potentially contribute to Evergreen’s carbon
neutrality goals. This thesis evaluates the performance of the flat plate and evacuated tube
collectors currently providing hot water and space heating to Mod 303. In the following
sections, common systems and components of solar thermal heating will be examined to
develop a necessary understanding of the technology which this research and the
Comparative Solar Project revolve around. This will lead into an explanation of the
methodology chosen for analyzing the performance of each collector as well as the many
variables that affect the outcome of this study. A discussion of significant findings will
precede the final section of this thesis, which will provide monitoring and data
management recommendations for the next year of operation.
6
SOLAR THERMAL TECHNOLOGY
It is important to this research to understand how solar thermal hot water heating
works and the many aspects of this technology. Many types of solar thermal collectors
and systems exist, providing varying benefits and drawbacks depending on the site,
climate and individual application needs. This section of the thesis will review the basics
of solar thermal technology and will describe the system installed on Evergreen’s Mod
303.
First, solar thermal is much different than solar photovoltaics. Solar photovoltaic
converts solar radiation into electricity while solar thermal heating uses the solar
radiation to directly heat a fluid, usually water. Solar radiation is the “rate of energy from
the sun being delivered to a surface at any given time” (Irradiance vs. Insolation).
Solar thermal systems, also called collectors, are most commonly used for heating
swimming pools, domestic hot water heating, space heating, industrial processes and
power production. Collectors generally consist of a frame, absorber, glazing, and
insulation (Machine-History.com). They convert solar radiation into thermal energy
which can be used immediately or stored for later use--most systems rely on storage tanks
to store and exchange heat. Many residential hot water systems, for example, utilize 80120 gallon hot water tanks while larger applications will utilize specialized, custom
storage options (Brehm et al. 6). Furthermore, collectors can be categorized into three
separate categories: low-temperature, medium-temperature, and high-temperature, as
described below.
Low-temperature collectors provide low-grade heat and are usually unglazed
flat plate collectors capable of heating water up to 120° F. This makes them ideal for
7
heating outdoor swimming pools, which is the number one use of solar thermal energy in
the United States. Unglazed flat plate collectors can provide up to 80% - 90% of the
energy needed to heat a typical residential pool and offer a cost competitive alternative to
natural gas and electric water heaters (Basu and Klenck). These collectors are relatively
simple and generally cost between $3,000 and $4,000 to purchase and install. Depending
on local fuel costs, these systems can pay for themselves within one and a half to seven
years (Solar Swimming Pool Heater). The design includes a “black plastic absorber with
flow passages, no glass cover; no insulation, and no expensive materials such as
aluminum or copper” (Brehm et al. 3). These collectors can effectively use solar thermal
energy in warmer climates although they lose efficiency in colder climates.
Medium-temperature collectors provide medium-grade heat between 140° 180° F and are commonly used for domestic hot water heating and space heating.
Medium-temperature collectors may also be used in industrial heating. These collectors
have the potential to provide up to 50% of the domestic hot water heating demand in the
United States (Solar Thermal: Energy Place). Glazed flat plate collectors (see Figure 3)
are the most common medium-temperature collectors. They consist of an insulated,
weatherproof box which contains a dark absorber that absorbs solar radiation and
transfers thermal energy to a fluid circulating through the panel. Other uses of glazed flat
plates include drying crops and heating indoor swimming pools.
8
Figure 3: Flat Plate Collector. "Solar Heating and Cooling Technologies."
United States Environmental Protection Agency. Environmental Protection
Agency, 8 May 2015. Web. 11 Aug. 2015.
Evacuated tube collectors fall into the medium-temperature category as well as
the high-temperature category. Evacuated tube collectors are often used in overcast and
colder climates. The collector consists of rows of parallel glass tubes. The air between the
outer and inner tube is vacuumed out in order to reduce convection and conduction heat
loss, which allows evacuated tube collectors to reach higher levels of efficiency than flat
plate collectors (see Figure 4). Evacuated tube collectors work better with higher
absorber temperatures and low radiation, exhibiting greater efficiency in the morning and
afternoon when the sun’s angle is 40° - 80° from perpendicular. This feature lets these
collectors obtain higher heat output throughout the day (Machine-History.com).
9
Figure 4: Evacuate Tube Solar Collector. "Solar Heating and Cooling
Technologies." United States Environmental Protection Agency.
Environmental Protection Agency, 8 May 2015. Web. 11 Aug. 2015.
High-temperature collectors heat water above 180° F and are used for industrial
processes and electricity generation. Evacuated tube and highly efficient glazed flat plate
collectors can fit into this category. However, concentrating solar collectors such as
parabolic troughs and dishes are the most effective at reaching high temperatures, and can
reach temperatures much higher than 200° F. In a concentrated solar collector system,
mirrored dishes or troughs focus the sun’s energy on an absorber filled with a heattransfer liquid or water (see Figure 5). Concentrated collectors are mainly used for power
production in regions with high solar radiation resources (Solar Heating and Cooling
Technologies).
10
Figure 5: Concentrating Solar Collector. "Solar Heating and Cooling
Technologies” United States Environmental Protection Agency.
Environmental Protection Agency, 8 May 2015. Web. 11 Aug. 2015.
Solar thermal systems can be further categorized into various configuration
options: passive and active, direct and indirect. Passive systems do not require pumps or
other electrical operating components. In an active system, heat exchange relies on a
circulating pump and temperature regulation apparatus. Direct (open loop) systems
circulate potable water directly through the system while indirect (closed loop) systems
circulate a heat-transfer liquid between the collector and storage tank. Figure 6 compares
common characteristics of the different system types.
11
Figure 6. Solar Hot Water System-Types Compared. Marken, Chuck. "Solar Water Heating
Systems Buyer's Guide." Home Power. Home Power Inc., June-July 2008. Web. 22 July 2015.
Passive systems work well in warmer climates with little risk of freezing, relying
on convection or gravity to circulate water instead of pumps or other electrical
components. The water circulated through the collectors is the same water being
delivered directly to the user. There are two types of passive systems: thermosyphon (see
Figure 7) and integral collector storage (see Figure 8).
12
Figure 7: Thermosyphon System. “System Types.” Florida Solar Energy Center.
N.p., n.d. Web. 2 May 2015.
Thermosyphon systems accomplish fluid circulation by convection and do not
require ancillary pumps in order to circulate. Water tanks are elevated above the collector
and the cold water in the bottom of the storage tank falls down through the solar
collector. After being heated, the water rises back up through the system in a separate
line, and settles on top in the storage tank (Machine-History.com).
13
Figure 8: Integral Collector Storage. "Integral Collector Storage Solar Thermal
System Layout." Solar365. N.p., n.d. Web. 19 July 2015.
Integral collector storage systems (ICS), combine the solar collector and hot
water storage and are generally used in combination with a conventional water heater.
These simple systems provide reliable preheating. The tank is housed in an insulated box
that absorbs the sun’s energy and heats the water. As hot water is withdrawn for use, cold
water flows into the tank and settles at the bottom, gradually rising to the top as it heats
up (Machine-History.com).
Active systems utilize pumps or other electrical components to circulate water or
a heat transfer liquid through the system. This allows active systems to operate more
efficiently than passive systems, but also introduces additional electricity costs. Unable to
function during power outages, active systems require a source of backup power such as
14
a generator or photovoltaic circulator (Brehm et al.7). An active system can also be direct
(open loop) or indirect (closed loop).
Direct or “open loop” systems circulate potable water from the hot water storage
tank directly through the collector. Direct systems contain temperature sensors that
monitor the temperature of the water inside of the collector and the water inside the
storage tank. When temperatures inside the collector exceed temperatures inside the tank,
the sensors activate a pump which circulates water through the system, replacing colder
water in the tank with hot water from the collectors. These systems are usually installed
in warmer climates that do not experience freezing temperatures (Solar Hot Water
Heaters Active Systems).
Indirect or “closed loop” systems utilize a heat-transfer liquid such as glycol
anti-freeze or distilled water and are suitable for climates with freezing conditions. In
these systems, the heat-transfer liquid is piped through the collector and returned to the
hot water storage tank where heat exchangers transfer heat from the heat-transfer fluid to
the water inside the storage tank (Brehm et al. 8).
Drainback systems are usually indirect but can also be direct in some cases
(Machine-History.com). These systems use either water or a heat-transfer liquid and a
pump to move fluid from the storage tank to the collector. Gravity helps the liquid to
drainback into the storage tank from the collectors. Drainback systems come fitted with
sensors that will allow liquid to drain from the collectors once temperatures in the
collectors fall beneath a predetermined level. Since no water remains inside of the
collector once temperatures fall, drainback systems provide excellent protection against
freezing in colder climates (Shelton).
15
Evergreen’s evacuated tube and flat plate solar thermal combi-system supplies
Modular Housing Unit 303 (Mod 303) with domestic hot water and radiant heated floors.
It is an active indirect system (see Figure 9), consisting of two different types of solar
Figure 9. Active, Closed Loop Solar Water Heater. "Solar Water Heaters."
Energy.gov. U.S. Department of Energy, 14 Dec. 2014. Web. 3 May. 2015.
thermal collectors: an Apricus AP-30 evacuated tube collector and a Caleffi NAS10410
flat plate collector. Each collector is designed to produce up to 42,000 Btu’s per day and
the two work in concert to provide as much of the required water and space heating as
possible for the eight residents occupying Mod 303.
The Apricus AP-30 is an evacuated tube solar collector “ideal for residential
households of 4-5 people, able to provide 60 - 80% of domestic hot water demand” (AP30 Solar Collector). The AP-30 consists of 30 evacuated tubes, filled with a specialized
fluid. As heat builds up, this fluid vaporizes and travels towards the heat exchanger where
the heat energy transfers to the propylene glycol flowing through the collector. The
16
glycol is then pumped into the hot water storage tank via electric pumps before returning
to the evacuated tube collector.
The Caleffi NAS10410 is a flat plate collector, made with a copper absorber and
covered with selective crystal coating capable of absorbing short-wave light while
reflecting long-wave light (Selective Surface). This results in increased efficiency for the
system. The absorber transfers heat to the flow tubes within the collector, which contain
propylene glycol, and the glycol is pumped into the hot water tanks that serve Mod 303.
The two hot water tanks in the solar shed attached to the side of the modular
housing unit each receive glycol from their individual collectors. The glycol is pumped
from each collector and heads into one of two tanks containing cold water from the
domestic mainline. The glycol flows through copper tubing running through the tank and
transfers heat to the water within the hot water tank. The glycol is then pumped back up
to the collectors, while the heated water is sent to the domestic hot water supply or the
closed loop radiant floor system. Before reaching the end-use destination, hot water will
pass through two tankless hot water heaters, which measure temperature and provide an
extra heat boost if necessary. The tankless hot water heaters are also connected to a
propane backup to provide continuous hot water to the modular in the event of a power
outage or other emergency.
The flat plate and evacuated tube collector have each been outfitted with Caleffi
iSolar BX Differential Temperature Controllers. The iSolar BX controllers control the
system functions of the collectors. The controllers have both been set at a minimum
operating temperature of 80° F. If glycol within the collectors does not reach this
minimum temperature, the pumps will not turn on and begin circulating fluid through the
17
system. The operating differential is set at 18/5. The collectors must be at least 18° F
warmer than the hot water storage tank before turning on and will shut off when the
temperature differential drops to 5° F (Solar Handbook 2). The controllers also send
collector temperatures to a secure digital (SD) card. Solar thermal collector data for this
research was obtained from each controller’s SD card.
18
METHODS
Data Sources
The two primary data sets utilized in this research measured the temperature of
the glycol within the collectors on Evergreen’s Mod 303. Each collector and the
associated hot water tanks have been fitted with an iSolar BX Differential Temperature
Controller that saves the information to an SD card; that data can then be analyzed using
computer software. The measurements include the date, time and glycol temperature in
the collector. Temperature analyzed in this thesis was recorded in ten minute intervals
from May 1, 2014 to May 27, 2015. Examination of glycol temperature data provides a
basic and straightforward approach to evaluating the heating ability of the collectors and
determining how well they function on this specific site.
Average hourly solar radiation and air temperature data served as the secondary
sets of data for this research to assist in visualizing trends between hot water production
and the amount of solar radiation reaching the collectors throughout the hours, days and
months. Solar radiation and average air temperature were obtained from the Washington
State University Agricultural Weather Network’s southwest Tumwater location,
approximately 8.5 miles south from the Evergreen State College. Comprised of 166
monitoring stations throughout Washington, AgWeatherNet measures many different
climate variables. The solar radiation and average air temperature data records every five
seconds and is summarized every fifteen minutes (AgWeatherNet). The southwest
Tumwater site was chosen because of its proximity to the Evergreen State College and
the wide range of available data.
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Data Preparation
A combined date and time stamp for the collector temperature originally appeared
in the same cell when the data were downloaded into Microsoft Excel. The first step
involved separating day, month, year, and time so that the data could be filtered and
analyzed more easily. Next, a new column was created, Hour, which categorized time
from 1:00 to 24:00. Each of the six temperature measurements that occurred every hour
were averaged into one hourly value, producing 24 measurements per day. This method
was utilized to condense the data into manageable units and to achieve compatible hourly
measurements with other data used for this research. Some hourly averages were not
available due to the system being offline as a result of maintenance or when the SD card
had been removed for data transfer. These cells were left blank and were not considered
during analysis since there was no reliable method to estimate their values.
In parallel, solar radiation and air temperature data from AgWeatherNet was
summarized hourly upon retrieval. Occasionally, the hours of 12:00 AM and 1:00 AM
had not been included in the downloadable spreadsheets. Aligning this data with the
hourly collector temperature data made it possible to identify missing values by filtering
12:00 AM and 1:00 AM in Excel, revealing each day that these hours were absent. Upon
identification, these hours were manually included in the data. Solar radiation values for
these times remained zero since there is no solar radiation in the early morning. Missing
values for average air temperature at 12:00 AM and/or 1:00 AM were replaced by
averaging the previous and next available hour’s temperatures.
Data from May, 2014 to November, 2014 were recorded in Celsius in Microsoft
Excel. Data from March, 2015 to May, 2015 did not have an indication of Celsius or
20
Fahrenheit. Since, average hourly air temperature data was utilized to look for a
relationship between collector temperatures, that designation was important. Collectors
would naturally reach air temperature after cooling down at night. One year of air
temperature data was compared to average hourly collector temperatures to conclude that
the unit was indeed Celsius. All temperature data was then converted to Fahrenheit to
maintain consistency with the air temperature data and multiple system settings that
already utilized Fahrenheit.
Identifying Research Limitations
The data available on the solar thermal collectors is extremely limited. Since an
advanced monitoring system has not been installed, only one variable has been measured:
glycol temperature within the collectors. The absence of additional data prevents further
investigation of:
System efficiency,
Cost-benefit analysis,
In-depth collector performance, or
Carbon offset potential.
However, temperature data from each collector allows for a generalized analysis focusing
on the ability of each collector to heat glycol. Heating trends can be identified and
compared between collectors, as can how each collector roughly responds to solar
radiation. This analysis relies on the assumption that higher temperatures indicate
increased hot water production, although after a point, this can also indicate system
malfunctions associated with overheating.
One significant limitation in particular is the operating temperature differential,
which presents uncertainty into any analysis of the available data. The operating
temperature differential introduces extra requirements that regulate the operation of the
21
pumps. First and foremost, the pumps will not turn on until the fluid in the collectors
reach 80° F. However, the system is set at an 18/5 operating differential, meaning the
pumps will not turn on at a collector temperature of 80° F unless the collectors are at least
18° F warmer than the water within the hot water tank. Furthermore, the pumps will not
turn off until fluid temperature within the collector drops to 5° F below the water in the
hot water tank. Although 80° F is the minimum collector temperature needed to activate
the system, the lack of hot water tank temperature data makes it impossible to determine
when the collectors were 18° F warmer or 5° F colder than the storage tanks. This means
that during any times when the collector temperature reaches 80° F, the pumps may not
actually turn on if the operating differential is not correct. Because of this, the following
analysis does not consider operating differentials when evaluating solar thermal collector
performance. The analysis focuses on collector temperatures above 80° F when pumps
had the potential to begin circulating the heat transfer liquid, propylene glycol, if the
operating differentials were in the ideal operating ranges.
Temperature measurements for the evacuated tube collector began on May 1st,
2014 and continued through the end of December. Measurements began for the flat plate
collector on July 29th, 2014 and lasted through August 8th, 2014, at which point the
system was taken offline due to abnormally high collector temperatures. Data collection
resumed on September 3rd, 2014 and lasted through the end of December of that year.
Neither collector operated during January or February, 2015 while RAD and
South Sound Solar employees evaluated the cause of irregular temperature readings and
performed system maintenance. It appeared that the system had been delivering
inaccurate temperature readings most likely due to pump cavitation. Cavitation occurs
22
when bubbles or cavities form in areas of low pressure and implode around the impeller
of a pump (Pump Cavitation Causes). Eventually, the force of these implosions begins to
damage the pump and other surrounding components. There are several causes of pump
cavitation:
A drop in pressure at the suction nozzle,
Increase of the temperature of the liquid being pumped,
Increase in fluid velocity at pump suction,
Reduction of the flow at pump suction,
Undesirable flow conditions caused by obstructions or sharp elbows in piping,
and
Positive suction head requirements are not met.
In this case, it appeared that the system had been improperly commissioned and
air had been trapped within the pipes. The pumps cavitated and caused the system to
stagnate, or overheat. Stagnation is often a problem during the summer months when
solar radiation and ambient air temperatures are high (Harrison, Lin, Mesquita 1). During
the summer, hot water demand, and especially radiant heating, is low, which often results
in overheating. Since the high temperatures will trigger an emergency shutoff of the
pumps, the collector continues to heat up the heat transfer liquid. The liquid expands and
can eventually turn into steam, causing a host of issues and potential damage to the many
components of the system (Hausner, Fink 4).
It appears that stagnation may have happened in July, August and September but
no conclusion has been reached as of this writing as to when exactly this happened or for
how long any underlying issues may have influenced the data being collected. The glycol
was flushed and replaced before temperature monitoring resumed on March 1, 2015.
They have continued to operate and collect data since that date.
23
As a result of the issues just outlined, the temperature data sets are best used to
gain insight into the first year of operation of the solar thermal collectors. It is important
to recognize that inaccurate data entries are most likely due to overheating and false
readings from pump cavitation.
Temperature Analysis
Knowing that the pumps begin circulating glycol through the collectors when
temperatures reach 80° F, the total hourly temperatures equal to or greater than 80° F
were identified and categorized as times when the system actively produced hot water for
circulation throughout Mod 303. Any temperatures that reached the emergency shutdown
limit of 270° F were noted as times when the collectors did not produce useable hot
water. Counting the number of hours in each category revealed how many hours each
collector actively fed hot water to Mod 303 as well as the percentage of hot water
producing hours during the data collection period.
Average hourly temperatures equal to or greater than 325° F were also identified
due to the specific brand of propylene glycol used in the collectors. Both collectors
contain Heliodyne DYN-O-FLO propylene glycol, which can reach a maximum
temperature of 325° F with a periodic usage rating of 375° F (Heliodyne 5).
Temperatures in excess of 325° F can cause the system to stagnate and result in
component erosion (Ramlow). Therefore, recognizing periods when the collectors
overheat is very important because these occurrences have the potential to cause
permanent damage to the system.
The flat plate collector data required additional analysis of the ten minute interval
measurements due to unusually high temperatures in the data set. This analysis consisted
24
of pinpointing the exact times the flat plate collector reached temperatures high enough to
trigger an emergency shutoff or cause glycol degradation. Since the hourly averages
could be significantly influenced by one extremely high temperature measurement, it was
necessary to look at the uncondensed data to gain a deeper understanding of the
occurrence of temperature spikes.
25
RESULTS
The results of this study represent a preliminary analysis of the first operating
year of the Evergreen Mod 303 flat plate and evacuated tube solar thermal collectors. The
following information will provide an overview of the basic ability of each collector to
heat glycol that is then pumped through solar hot water tanks as a heat transfer liquid.
The findings of this research will supplement the available documentation on this project.
These results can also assist individuals working with the solar thermal system and serve
as a basis for comparison of future data collection. The first section that follows will
evaluate collector temperature throughout the year, considering times when the system is
active, inactive and operating beyond normal specifications. The second section explores
the flat plat collector’s emergency shutoff incidence rate, which was noticeably high
throughout the year. The final section provides a brief summary of the findings after the
emergency shutoff issues have been addressed and factored into the collector’s
performance.
Collector Temperature Analysis
As indicated earlier, all average hourly temperature measurements equal to or
greater than 80° F turn on the pumps that circulate glycol through the solar thermal
collectors. Hourly temperature was calculated to find the ratio between hours when the
system started up and provided hot water to Mod 303 and hours when the system
remained inactive. The values are presented in Table 1. The evacuated tube collector
initiated the pumps 2,948 hours out of 7,912 total hours of online operation from May 1,
2014 to May 27, 2015. The flat plate collector initiated the pumps more frequently with
2,592 hours out of 5,146 total hours of online operation but reached temperatures that
26
triggered the emergency shut off for a total of 142 hours, lowering the heat producing
hours to 2,450. The flat plate collector supplies heat approximately 47.6% of the time,
while the evacuated tube collector supplies heat approximately 37.3% of the time. This
indicates that the flat plate collector has the potential to supply more hot water than the
evacuated tube as long as it does not reach temperatures that will stop the pumps and hot
water delivery too often. Although it appears that emergency shutoffs do not stop the flat
plate from outperforming the evacuated tube, further observation will be needed to
determine if this is actually the case. This will be particularly important through the
months of June, July and August, when flat plate performance data was either extremely
limited or not being collected.
Total Hours
Hours ≥ 80° F
Hours ≥ 270° F
Active Hours
Inactive Hours
Time Spent Actively
Producing Heat
Evacuated Tube Flat plate
7,912
5,146
2,948
2,592
0
142
2,948
2,450
4,964
2,696
37.3%
47.6%
Table 1. Total Recorded Hours vs. Active Recorded Hours.
Average daily collector temperatures for each month varied between the
collectors (see Table 2). The evacuated tube had a low daily average temperature of 50.2°
F in November and a high daily average of 90.7° F in August. The flat plate exhibited a
much larger range of a low daily average of 55° F in November and a high daily average
of 240.8° F in July. Note, however, that the flat plate temperatures were only recorded for
three days in July and eight days in August. This significantly limits the amount of
27
measurements available to calculate an average daily temperature for either of these
months. Graphical representation of this data is presented in Figure 10.
Average Monthly
Temperature and Solar
Radiation
Month
Year
May
June
July
August
September
October
November
December
January
February
March
April
May
2014
2014
2014
2014
2014
2014
2014
2014
2015
2015
2015
2015
2015
Evacuated Flat-Plate Solar Radiation
Tube °F
°F
W/m²
81.4
84.5
88.4
90.7
84.2
69.9
55
50.2
NA
NA
68.7
74.3
81.2
NA
NA
240.8
228.5
161.8
99.5
55
48.5
NA
NA
83.1
95.9
97.1
340
343.1
397.8
355.3
293.1
177.5
127.4
87.9
94.5
157.3
231.9
318.4
325.3
Table 2: Average Collector Temperatures and Solar Radiation per Month.
Figure 10. Average Monthly Collector Temperatures.
28
Figure 10 presents an interesting view of the heating and solar radiation trends
from the beginning of the data collection period to the end. Note that the flat plate
collector exhibited extremely high temperatures in July, August and September before
reaching temperatures comparable to the evacuated tube collector through the end of the
year, when solar radiation dropped significantly. Once data collection resumed in March,
the flat plate collector produced temperatures much closer to the evacuated tube and did
not reach the extremely high temperatures it had previously reached during August and
September, despite comparable solar radiation (Figure 10). After system maintenance in
January and February, which included a glycol flush and refill, it appeared the flat plate
collector began working correctly and staying within safe operational temperatures. After
stagnation, glycol begins to lose its effectiveness and can overheat more frequently and
lose its antifreeze properties (Ruxton). A flush and refill of the system with new glycol is
most likely the reason for a reduction in temperature spikes. However, it will be
important to closely observe this collector in the future in case another unidentified issue
is affecting heat production.
Interestingly, temperatures of the evacuated tube collector remained relatively flat
throughout the year in comparison to the flat plate collector. Evacuated tube collectors
are generally more efficient and capable of reaching higher temperatures than flat plates
(Williams) yet it did not produce comparable heat until November and December.
Evacuated tube collectors perform better in cold climates because they lose very little
heat energy due to the vacuum of the tubes (Flat Plate versus Evacuated Tube
Collectors). Although both types of collectors are very well suited for domestic hot water
29
production, it comes as a surprise to discover the flat plate reaching consistently higher
temperatures than the evacuated tube.
The following graphs depict average daily collector temperatures for each month
(Fig. 11-21). These graphs are designed to serve as a visual representation of the general
performance of both collectors. Each description includes information on average hourly
temperatures as well, which are not practical to view in the context of this thesis. Solar
radiation has been included to assist with the visualization of seasonal radiation and
change in collector performance.
Figure 11. May 2014 Average Daily Collector Temperature and Solar Radiation.
During the first month of data collection only the evacuated tube collector was
online and sending collector temperature measurements to the iSolar BX controller. Data
collection began on the first and continued throughout the rest of the month. Out of 744
30
hours recorded, the evacuated tube reached temperatures greater than or equal to 80° F
for 353 hours, or approximately 47% of the time. It did not rise above 270° F.
Figure 12. June 2014 Average Daily Collector Temperature and Solar Radiation.
Throughout June, the evacuated tube collector reached 80° F for 372 hours out of
the 720 recorded, or approximately 52% of the time. Once again it did not reach
temperatures above 270° F. Heat production remained relatively consistent even when
solar radiation was high.
31
Figure 13. July 2014 Average Daily Collector Temperature and Solar Radiation.
The first temperature collector measurements for the flat plate collector occurred
in July on the 29th, 30th and 31st. The flat plate collector reached temperatures higher than
80° F for all of the 65 recorded hours. Temperatures above 270° F totaled 20 hours as
well as 13 hours of temperatures in excess of 325° F. The evacuated tube continued to
steadily produce heat with minor response to fluctuations in solar radiation.
The evacuated tube collector continued operating as it did in May and June,
reaching temperatures above 80° F for a total of 374 hours, or 50% of the time. No
temperature measurements reached above 270° F.
32
Figure 14. August 2014 Average Daily Collector Temperature and Solar Radiation.
Temperature measurements occurred for the first eight days of August at the flat
plate collector. Yet again, 100% of the 182 recorded hours reached temperatures above
80° F while a total of 50 hours exceeded 270° F and 30 hours exceeded 325° F. The flat
plate collector stayed within productive temperature ranges for 69% of the recorded
hours. The flat plate’s drastic fluctuation in temperature, apparent in Figure 14, is much
different than the response seen from the evacuated tube collector during the past three
months.
The evacuated tube collector maintained operational temperatures above 80° F for
approximately 50% of the 744 total hours. Collector temperature continued to remain
steady as solar radiation levels fluctuated.
33
Figure 15. September 2014 Average Daily Collector Temperature and Solar Radiation.
Data collection for the evacuated tube collector occurred throughout September
when solar radiation began to noticeably decrease. The evacuated tube reached
temperatures above 80° F for approximately 43% of the recorded hours, the lowest
percentage of active hours since monitoring began in May. It did not exceed temperatures
above 270° F.
The flat plate came back online on September 3rd and continued through the end
of the month. Out of 659 total hours, the flat plate reached temperatures above 80° F
approximately 90% of the time. Average daily temperatures of the systems did not reach
above 270° F. However, on an hourly basis approximately 9% of recorded hours were
above 270° F and 5% were above 325° F. The first full month of flat plate monitoring
revealed that this collector tracked solar radiation quite well, generally increasing and
decreasing in temperature along with available solar radiation. This is unexpected since
34
evacuated tube collectors generally outperform flat plate collectors. This may be an
indication that one of the collectors was not functioning properly during the past several
months: either the flat plate was overheating throughout the month (which it was,
although how severely compared to normal stagnation occurrences is not known) or the
evacuated tube was not absorbing a sufficient amount of solar radiation. Both of these
scenarios may have been taking place simultaneously.
Figure 16. October 2014 Average Daily Collector Temperature and Solar Radiation.
Both collectors were in operation throughout October. The evacuated tube
exceeded temperatures of 80° F for 30% of the 744 total hours recorded, compared to
43% the previous month. Once again, no hourly temperatures triggered an emergency
shutoff. Heating capability clearly decreased along with solar radiation during October.
Flat plate performance decreased significantly in October, reaching temperatures
greater than or equal to 80° F for approximately 62% of 744 total hours. Temperatures
35
had exceeded 80° F 90% of the time during the previous month. However, in October,
only eight hours exceeded 270° F and three exceeded 325° F, or 1% and 0.4% of the total
recorded hours. The system began to overheat significantly less as daily solar radiation
continued to fall.
Figure 17. November 2014 Average Daily Collector Temperature and Solar Radiation.
During November, the evacuated tube collector did not reach average daily
temperatures above 80° F and only reached average hourly temperatures above 80° F
12% of the time. No temperatures exceeding 270° F were recorded. These measurements
are much different than those recorded during the summer months, when the evacuated
tube consistently exceeded 80° F for approximately 50% of all recorded hours.
Average daily temperatures of the flat plate collector exceeded 80° F twice, and
did not exceed 270° F. On an hourly basis, temperatures reached 80° F or higher for
approximately 9% of the 708 recorded hours. No Temperatures exceeded 270° F.
36
November was the first month that the flat plate did not outperform the evacuated tube
collector. Flat plate collectors are very susceptible to ambient air temperature since it can
significantly affect their ability to produce heat. Unlike evacuated tubes, which hold in
heat extremely well due to their vacuum design, flat plate collectors lose heat to the
surrounding atmosphere as temperatures drop. Flat plate collectors also rely more on
direct sunlight than evacuated tube collectors, which can absorb indirect light through the
different angles of the tube. During the fall and winter months, temperatures drop sharply
along with available solar radiation, which better suits the evacuated tube collector and
leaves the flat plate experiencing a much more significant performance decrease.
Figure 18. December 2014 Average Daily Collector Temperature and Solar Radiation.
In December, average daily temperatures for both collectors did not reach 80° F. The
evacuated tube exceeded 80° F on an hourly basis only 24 times, or only 3% of the 744
37
hours recorded. Again, the flat plate produced less heat than the evacuated tube reaching
temperatures above 80° F only 10 times, or approximately 1% of the 744 hours recorded.
Figure 19. March 2015 Average Daily Collector Temperature and Solar Radiation.
After data collection resumed on March 1st, both collector’s average temperatures
began to rise again. Daily temperatures for the evacuated tube rose above 80° F twice at
the end of March, and average hourly temperatures exceeded 80° F for 239 hours out of
735 recorded, approximately 33% of the time. No daily or hour temperatures reached
emergency shutoff limits.
The flat plate collector began to outperform the evacuated tube once again,
reaching average hourly temperatures for 349 out of 734 hours recorded, or
approximately 48% of the time. Emergency shutoff was only triggered for one hour
during March. This suggests that the system maintenance and glycol flush that occurred
during January – February may have had a role in preventing incidences of overheating.
38
Figure 20. April 2015 Average Daily Collector Temperature and Solar Radiation.
In April, the flat plate collector continued to outperform the evacuated tube
collector. The flat plate exceeded 80° F approximately 62% of the time for 431 of the 690
recorded hourly averages. Once again, emergency shutoff limits were only recorded
during one hour in that month. The flat plate also began to respond to fluctuations in solar
radiation much like it did during the previous summer and fall. Since the flat plate loses
heat more easily than the evacuated tube, a larger range would exist between low and
high temperatures due to cooling during times of low ambient air temperature and solar
radiation.
The evacuated tube collector increased heat production over March, initiating the
pumps at 80° F or more for 276 of the 686 recorded hourly averages, or 40% of the time.
Performance almost returned to the previous summer’s average heat production of
approximately 50% and continued to not exceed 270° F. Lower heat production from this
39
collector remains an unexpected observation due to the evacuated tube’s ability to absorb
more light from multiple angles. This should be noted for further consideration in future
analysis.
Figure 21. May 2015 Average Daily Collector Temperature and Solar Radiation.
As daily average solar radiation approached levels comparable to the previous
August and September, the flat plate still did not reach emergency shutoff limits on a
daily basis. Production continued to increase, exceeding 80° F for approximately 70% of
the 620 recorded hourly averages, a total of 436 hours. Emergency shutoff temperatures
were not reached on an hourly basis. Since being serviced, it appeared the flat plate
collector had been running within much safer temperature ranges. However, this collector
should be carefully monitored until next fall when solar radiation begins to drop once
again.
40
The evacuated tube maintained its flat temperature profile as solar radiation
continued to rise. It kept the pumps activated above 80° F for 47% of the 620 recorded
hours, a total of 292 hours. No emergency shutoff limits were reached.
These monthly graphs clearly show how solar radiation influences collector
performance. As expected, months with higher levels of solar radiation exhibit increased
solar thermal production while months with lower solar radiation exhibit a decrease in
solar thermal production. The evacuated tube collector generally maintains consistent
heat production although it does not reach temperatures as high as the flat plate collector.
The flat plate collector is much more reactive to fluctuations in solar radiation and
demonstrated ongoing issues with overheating and triggering emergency shutoffs of the
system.
Emergency Shutoff Analysis
As discussed previously, when either collector reaches temperatures above 270° F
the pumps automatically shut down. This preserves the system and prevents overheating,
which can cause the pumps to cavitate and the glycol to eventually stagnate within the
collectors. It is important to identify temperatures equal to or greater than 270° F because
the pumps shut down and do not circulate fluid through the collectors during these times.
Temperatures higher than 270° F were included in the analysis since they represented
inactive periods of operation.
Evacuated tubes can reach temperatures in excess of 400° F, however, the Apricus
AP-30 did not reach temperatures this high during the year of data collection
(Wondrausch). Usually capable of reaching temperatures around 180° F, the Caleffi flat
plate installed on Mod 303 can handle temperatures up to 350° F (Caleffi Hydronic
41
Solutions). The flat plate reached temperatures well above the emergency shutoff point
numerous times during the year. Since both collectors are capable of reaching
temperatures that are higher than the emergency shutoff point and the glycol operating
limits, this analysis assumes that any temperatures above those two points are possible.
However, due to the lack of monitoring ability built into the system, it is not possible to
determine the difference between the maximum temperature reached and when false
measurements were being recorded due to stagnation and sensor failure.
Average hourly temperature calculations show that the evacuated tube collector
did not reach temperatures of 270° F or higher during the year. However, it did reach
temperatures above 270° F for a period of five consecutive ten minute measurements on
April 27th, 2015 between 12:30 pm and 1:10 pm. These temperatures triggered an
emergency shutoff for fifty minutes. Collector temperature did not exceed 370° F at any
point in time, therefore the glycol did not reach levels of stagnation.
The flat plate collector reached average hourly temperatures of 270° F or higher
for 142 hours during the year. Out of those 142 hours, 80 were above 325° F. The raw,
uncondensed data becomes increasingly significant at this point. Due to these findings, it
became clear that an evaluation of the ten minute measurements for each collector would
be necessary. Over 65,000 flat plate collector temperature measurements were recorded
at ten minute intervals during the year. The flat plate collector triggered an emergency
shutdown 939 times; 489 of these measured 325° F or above. Measurements reaching
270° F or higher are shown on the next page by month (Fig. 22).
42
Figure 22. Flat Plate Temperatures ≥ 270° F.
In Figure 22, there does not appear to be a direct relation between total monthly
measurements greater than or equal to 270° F and total monthly solar radiation. However,
only three days at the end of July are represented, a total of 385 ten minute
measurements. Of those, 116 triggered emergency shutoff. In August, temperatures
reached above 270° F 299 times out of eight recorded days or 1,086 total measurements.
If data was collected for the full months of July and August there would most likely be an
extremely high incidence of emergency shutoff. Data collection halted for the remainder
of August while the flat plate collector and temperature sensors were inspected, as these
sensors were the suspected cause of the high temperature measurements. However, when
demand is low and collector temperatures are high, stagnation can become a problem in
collectors and cause various operational issues such as:
Acceleration of the breakdown of sealants and fittings,
Pump cavitation,
Component stress due to pressure spikes and drops,
Acceleration of glycol decay causing,
Increased acidity of glycol,
43
System component corrosion, and
Loss of antifreeze properties allowing the system to freeze (Ruxton).
Since sensors generally give off high or false temperature readings to indicate a
system failure, this is a probable explanation for the unexpectedly high temperature
measurements. Another cause may have been faulty setup of the monitoring system,
which was a concern with both systems due to the obstacles encountered during the initial
commissioning.
Data collection resumed in September and continued through December, when
solar radiation began to significantly drop. The flat plate collector continued to reach high
temperatures during this time, although the frequency decreased along with solar
radiation. As mentioned above, monitoring of the flat plate collector did not occur during
January or February, 2015. Emergency shutoff was only triggered thirty times from
March 1st to May 27th and did not increase along with increasing solar radiation. Table 3
below displays total temperature measurements each month. It also displays the amount
of measurements greater than or equal to 270° F and the amount of time spent at 270° F
or above.
Month
July
August
September
October
November
December
January
February
March
April
May
Year ≥270° F ≥325° F
2014
2014
2014
2014
2014
2014
2015
2015
2015
2015
2015
116
299
433
54
7
0
0
0
16
9
5
72
158
230
19
7
0
0
0
5
2
1
Total Monthly
Measurements
326
1,086
3,945
4,460
4,243
4,458
0
0
4,402
4,125
3,713
≥270° F ≥325° F
35.58% 22.09%
27.53% 14.55%
10.98% 5.83%
1.21% 0.43%
0.16% 0.05%
0.00% 0.00%
0.00% 0.00%
0.00% 0.00%
0.36% 0.11%
0.22% 0.05%
0.13% 0.03%
Total Solar
Radiation W/m²
398
355
293
177
127
88
95
157
232
318
325
Table 3. Flat Plate Total Recorded Hours vs. Active Recorded Hours.
44
A close look at Table 3 reveals the strange relationship between excessive
temperatures and solar radiation. The small amount of measurements taken in July
exhibited the highest percentage of high temperatures, although this is difficult to gain
insight from due to the small sample size. The eight days of collection in August also
exhibit a relatively high percentage of high temperatures as well as a decrease in average
solar radiation. Beginning in September, the first month of full data collection, the
percentage decreases again along with solar radiation. From October through the end of
the year, frequency of high temperatures continued to decrease along with a decrease in
solar radiation until eventually reaching zero in December. December also had the lowest
average solar radiation.
The surprising observation takes place in March through May, when solar
radiation continues to increase, reaching levels comparable to the previous September. In
March, only 0.36% of the measurements are temperatures capable of triggering an
emergency shutoff and this number continues to decline in the following months while
solar radiation increases. This suggests that the routine system maintenance and glycol
flush during January and February may have resolved any lingering issues from the
previous summer’s stagnation and cavitation events. However, it is still early enough in
the year that overall solar radiation and ambient air temperature may not be high enough
to cause another stagnation event. This will be an interesting observation to keep in mind
during next year’s analysis.
Collector Performance Summary
The evacuated tube collector performed without any noticeable issues. Data
collection occurred approximately 90% of the year. Average hourly collector
45
temperatures ranged from 20.7° F to 213.4° F throughout the year, which fall within an
acceptable range. The collector reached temperatures that triggered an emergency shutoff
for a total of fifty consecutive minutes during one hour of the year but did not reach
levels above 325° F. Overall, of 7,912 average hourly temperatures collected, 2,948 hours
show active hot water delivery to Mod 303 hot water tanks.
The flat plate collector reached operating temperature more often than the
evacuated tube, but also triggered emergency shutoff and reached temperatures that can
cause glycol degradation if sustained for extended periods of time. Data collection
occurred approximately 59% of the year. Average hourly collector temperatures ranged
from 20.5° F to 1248.1° F. Out of 5,146 average hourly temperatures, the flat plate
collector fell within normal operating temperatures for 2,450 hours. A total of 142 hours
reached temperatures above 270° F and caused an emergency shutdown, with 80 of these
hours exceeding temperatures of 325° F. After being adjusted for actual ten minute
interval data, total time lost due to emergency shutoff equals 156.5 hours, an increase of
14.5 hours over the average hourly estimate. 81.5 hours exceeded temperatures of 325° F.
Validity of the Results
Due to the limitations and obstacles that were identified while working with
available data, all results could potentially have been influenced by basic performance
assumptions, false temperature measurements (due to overheating and unidentified
factors) and other generally unknown variables and system malfunctions. These obstacles
did serve a purpose, as they shed light on specific problems that will require action in
order to fully understand this specific project.
46
DISCUSSION
After being awarded the grant on September 12, 2012, installation of the solar
thermal collectors began. Through a collaborative and creative effort, various individuals
stepped in to finish commissioning the system and keep it operating throughout the year.
Through the changing of hands, information related to maintenance, adjustments and
general operation has naturally slipped through the cracks and created uncertainties and
operational issues regarding the system. As a result,
Documentation on the design, construction and initial setup of the project is
limited;
Detailed records of maintenance, iSolar BX Differential Temperature Controller
settings and data management methods are difficult to come by;
There are uncertainties concerning basic system operations; and
Much of the information needed for in-depth analysis is lacking.
These issues have the potential to significantly influence the results of this analysis and
future analyses. As stated earlier, it is not possible to perform the following analyses:
System efficiency,
Cost-benefit analysis,
In-depth collector performance, or
Carbon offset potential.
Did the Comparative Solar Project Meet the Specified Goals?
Once again, the purpose of the Comparative Solar Project was:
● To install two different kinds of solar panels with similar collection capacities for
the purpose of comparison,
● To develop relationships between Residential and Dining Sustainability Crew
student-employees, faculty, and local companies involved in sustainable design,
and
● To foster a conversation between the above mentioned groups in order to develop
an academic understanding of the possibilities presented by solar hydronic
systems in the Pacific Northwest.
Unfortunately, these goals have not yet been reached to their fullest potential, although
the process of fulfilling each of these goals has begun. Two different solar thermal
47
collectors are now installed and operating at Mod 303. A comparative analysis has been
performed to the fullest extent possible with the data and resources available. However,
the results of the analysis echo the limitations currently surrounding the system. It is
absolutely critical that additional variables be monitored in the future so that the next
analysis will be able to fully evaluate the system and yield concrete evidence on the
collector’s performance.
Relationships between Residential and Dining Sustainability Crew studentemployees, faculty, and local companies have been built throughout the process of
getting this project off the ground. Realistically, these relationships are just beginning to
be forged. A complete understanding of this system will take continuing dedication from
students, faculty and industry professionals. These groups will need support from each
other as the project moves along.
The Comparative Solar Thermal project has definitely opened up avenues for
conversation between these groups. There are many questions left to be asked and many
solutions to be found. Once this first solar thermal system has reached its fullest
operating potential and can be comprehensively analyzed, the involved parties will be
able to begin to explore possibilities of expansion and the future of solar thermal at
Evergreen.
Theoretical Savings and Carbon Offset Analysis
One easily accessible method for estimating electricity and carbon savings can
provide some insight into the potential impact this technology can have at Evergreen.
Caleffi and Apricus both provide collector output calculators for their individual products
online. Using this method, a rough estimate of annual energy output can be produced as a
48
general reference to the potential of these collectors. Each of these output calculators
takes into account average annual insolation level in kWh/m2/day and the solar collector
in question. The average annual insolation level selected by Caleffi for the Olympia area
is 3.69 kWh/m2/day. Apricus originally selected 3.62 kWh/m2/day for Seattle, but the
value was adjusted to 3.69 kWh/m2/day to maintain consistency with the Caleffi
calculator since this value was not adjustable on the Caleffi website. The Apricus AP-30
evacuated tube collector and the Caleffi NAS10410 flat plate collector were selected for
evaluation. After running the programs, it was determined that the average estimated
energy output of the Apricus AP-30 evacuated tube was 2,659 kWh (Collector Output
Calculator). The Caleffi NAS10410 had an estimated energy output of 1,497 kWh
(Caleffi Hydronic Solutions).
In 2014, Evergreen’s average electricity cost was $0.086/kWh after all charges
and renewable energy certificates were applied by Puget Sound Energy (Utility/Energy
Consumption by Fiscal Year 2014). At this rate, the Apricus AP-30 would save
Evergreen $228.67/year while the Caleffi NAS10410 would save $128.74/year, totaling
$357.41.
These estimates can also be used to determine the rough carbon offset potential of
each collector. Puget Sound Energy’s 2012 CO2 emissions profile from all generating
sources was 1.48 lbs/kWh (Lo 9-1). At an estimated annual energy output of 2,659 kWh,
the Apricus Ap-30 would offset approximately 3,935 pounds of CO2 emissions. In
comparison, the Caleffi NAS10410 would offset 2,216 pounds of CO2 emissions. For
comparison, these emissions savings are the equivalent of not burning 6.5 barrels of oil or
not driving over 6,600 miles (Greenhouse Gas Equivalencies Calculator).
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According to the online calculators, the Apricus AP-30 evacuated tube collector
will produce more hot water than the Caleffi NAS10410. There are 19 modular housing
units located at Evergreen. If the same system was installed on each unit, with a second
evacuated tube collector replacing the flat plate, there would be 38 evacuated tube
collectors delivering hot water to the modulars. Theoretically, this would save 101 MWh
of electricity and $13,581/year. In comparison, Evergreen spent $1,182,498 on electricity
and used 13,787 MWh in 2014 (Utility/Energy Consumption by Fiscal Year 2014).
Moving Forward with Solar Thermal Hot Water Heating at Evergreen
The theoretical analysis portrays the opposite of what has been observed from the
evacuated tube and flat plate collectors installed on Mod 303. According to the results of
this research, the Caleffi flat plate collector is the ideal choice for producing hot water at
Mod 303. The flat plate collector outperformed the evacuated tube collector throughout
the year, with the exception of December, when the evacuated tube operated a negligible
1.7° F warmer on average than the flat plate. However, when combined with the
theoretical analysis, these results, although very rough, indicate a need for further
research before claiming one type of collector outperforms the other. These
circumstances should be evaluated over the next year because they could drastically alter
the next steps in evaluating and possibly installing further solar thermal hot water at
Evergreen.
Enhanced monitoring systems and maintenance procedures are the key to fully
comprehending the potential role of solar thermal hot water at Evergreen. A new
monitoring system could provide details of all the variables necessary to perform the
analyses listed at the beginning of this chapter. These variables include:
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Hot water storage tank temperature,
Temperature of water entering the hot water tanks,
Temperature of heat transfer liquid inside of the collector,
Temperature of heat transfer liquid entering the hot water tanks,
Temperature of heat transfer liquid, and
Pump speed.
The ability to monitor these variables would significantly broaden the range of analyses
that could be performed. Heat loss through different components and stages of the system
could be identified and minimized, parameters could be adjusted for improved efficiency
and heat production and system failures or other issues would be much easier to diagnose
and correct.
Recording all maintenance procedures and operating specifications in a secure
and accessible location and format would also assist in future troubleshooting and
speculations. All current system settings should be recorded and updated when any
changes occur. Important details to keep record of include:
Type of glycol or heat transfer fluid,
Maintenance receipts,
Maintenance records,
Strange operational observations,
Times when system is offline,
System or component failure, and
Any other relevant observations and information directly related to system
performance and maintenance.
Communication of any changes and observations will be very important for the parties
involved. While working through this data it appeared that different individuals held
different pieces of information and varying perspectives about the solar thermal system.
This makes sense considering the difficulties encountered while commissioning the
system and dealing with the original company responsible for the installation. The
recommendations covered here can be added to the Solar Handbook as useful guidelines
51
when recording and saving relevant information regarding the solar thermal hot water
system.
52
CONCLUSION
This research delivers insight into the first operational year of the Comparative
Solar Project installed on Mod 303 at The Evergreen State College. The comparative
analysis performed on the Apricus AP-30 evacuated tube collector and the Caleffi
NAS10410 flat plate collector indicates that the Caleffi flat plate collector outperforms
the Apricus evacuated tube collector throughout the year with the exception of
December. Designed to perform better in cold and cloudy climates, the Apricus AP-30
produced temperatures averaging 1.7° F warmer than the Caleffi in December. The
results gathered from this research show a need for further evaluation of both collectors
over the next year in order to provide a fully informed recommendation on which solar
thermal collector is best suited for serving Mod 303.
Due to the limitations of available data, there was great difficulty determining
how well each collector actually performed. The Caleffi flat plate collector was plagued
with overheating and pump cavitation issues, which significantly affected the ability to
accurately judge the data collected by the temperature sensors. The Apricus evacuated
tube collector did not display any significant operational issues, but curiously produced
lower temperatures than the flat plate collector, which was unexpected since evacuated
tube collectors generally reach higher temperatures than flat plate collectors.
Most importantly, this research displays the need for advanced monitoring
resources and abilities. An extensive, comprehensive monitoring system will make
possible in depth analyses such as carbon offset potential, cost-benefit analyses, accurate
performance comparisons and system efficiency analyses. In order to fully appreciate the
initial $15,000 investment made in this project, it is highly recommended to seek and
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provide funding for an advanced monitoring system that will allow this project to reach
its fullest potential as a possible future source of renewable energy and guaranteed
collaborative medium at The Evergreen State College. Despite the unforeseen obstacles
encountered while commissioning this project, the Comparative Solar Thermal Project
prevented the release of up to 6,150 pounds of carbon emissions into the atmosphere over
the past year, providing a valuable contribution to the College’s efforts of reaching its
carbon neutrality goals.
54
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