Project data

This project examines two hot water heating systems that are commonly used on residential buildings; Solar Thermal Collectors and Air source Heat Pumps. A spreadsheet comparing factors such as system cost, efficiency, operating costs, maintenance and pollution has been created. Values in the spreadsheet can be changed to produce a comparison answer that is more closely based on any user's needs. Preselected options in the spreadsheet are based on stated assumptions and are the baseline for comparison. The conclusion of this project has determined that the optimum hot water heating system varies depending on water use needs. For low water consumption, heat pumps are more cost effective. For higher water consumption, solar thermal panels are the more economical choice. Both systems are effective at reducing the consumer's carbon footprint.

Group members: Adam Channal, Devan Hemmings, Ryan Kriken, Robert Duncan

## Background

A homeowner in coastal Humboldt County, CA has an aging domestic hot water system, and would like to replace it with a more environmentally responsible system. Basing his decision on a set of criteria, the homeowner will choose to install either a solar thermal water heating system or an air-source heat pump. Adam Channel, Robert Duncan, Devan Hemmings and Ryan Kriken have been hired as consultants to research different hot water systems and provide a comparison of these two technologies. The client has expressed a desire to know the prices of different systems, their efficiency, lifespan, buyback time, maintenance needs, and energy costs for using the system. To simplify the client's decision, a spreadsheet allowing for changes to various inputs has been created. This spreadsheet allows for changes to variables such as system manufacturer, climate data, household water use, and several other variables to tailor the system to the client's needs.

### What is a Heat Pump?

A heat pump is any device which moves heat from one location to another. Heat pumps operate much like a refrigerator, only in reverse. A refrigerator pumps a refrigerant through a compressor and transfers heat from inside the cooled space to the outside ambient air thus cooling the inside of the fridge. A heat pump on the other hand takes heat from the outside and uses it to heat the inside air or takes inside or outside air to heat water.


There are two types of heat pumps: ground source heat pumps, and air source heat pumps. Either of these types can be used to heat a home or to heat water or both. We chose to do our analysis on air source heat pumps.

Ground source heat pumps basically work by circulating refrigerant through underground pipes and back through a compressor where it is vaporized, and condensed causing it heat up and then the heat is transferred to a coil where it can be utilized for home heating or water heating purposes. The reason the pipes are run underground is to utilize the consistent 58 degree Fahrenheit ground temperature which in cold climates may be higher than the average air temperature. Also, many air source heat pumps become very inefficient below 40-50 degrees F and in very cold climates will stop working. Ground source heat pumps can also be used as air conditioners in the summer months and are more cost effective when used to heat and cool a home. An attachment to this type of system, called a desuperheater, can be attached to also provide the home with hot water. We did not do an analysis of the desuperheater due to the very high installation costs of ground source heat pumps.

Air source heat pumps can be used either to heat the air or water. For home heating uses there is generally an outside unit with a fan and condenser drawing heat from the outside air, and inside unit with a fan, condenser, compressor, and heat expansion valve, and pipes running in to circulate the heat to and from the inside environment.

For air source heat pump water heaters, the heat from the ambient air is pulled with a fan into the condenser and heat is transferred into the water tank and heats the water through a coiled heat exchanger. Because heat is being moved instead of generated, a heat pump water heater can be up to about 60% more efficient than traditional electric water heaters.

Air source heat pump water heaters come as either a prepackaged unit which includes a tank, and an electric component heating unit, or as a stand alone unit that can be hooked into a preexisting water heater tank. For our project we chose the GE hybrid heat pump water heater because it had the highest energy factor efficiency rating and the installation and unit cost was fairly low compared to units that don't come as a package.

Because a heat pump is taking heat from the air around it, Heat pump water heaters require installation in locations that remain in the 40º–90ºF (4.4º–32.2ºC) range year-round. Garages, cellars, laundry rooms, or basements are ideal locations for installation. It is not recommended, however for a unit to be installed in a heated room because they tend to cool the air around them. It is also best not to have a unit installed outside due to problems with weather corrosion, and temperatures below 40 F. It is recommended that at least 1,000 cubic feet (28.3 cubic meters) of air space is in the room around the water heater.

### What is Solar Thermal?

Solar thermal water systems use energy gathered from sunlight to heat water. Sunlight falls on collector panels which is then moved to a hot water tank similar to conventional tanks. Many types of solar thermal systems exist some of which are more adapted to small scale and therefore residential use. The systems examined below can be adapted to fit either small scale or large scale water needs. Other systems such as parabolic solar collectors tend to be used for large scale collection and are not examined in this project.

Almost all residential Solar Thermal systems fall in to one of the following two types of systems:

Open Loop systems take water from local water pipes, pass it through the solar thermal panels to then be used directly for hot water. A schematic of such a system can be found to the right.

Closed Loop systems circulate a coolant/water through the solar thermal panels and transfer the heat using a heat exchanger to your hot water tank. In this system, the water from the tap stays in your water heater while it is being heated and does not travel through the solar thermal panels. A schematic of this system can be found to the right.

The two collector panel technologies examined in this project are:

Evacuated Tube Collector are layered glass cylinders that absorb sunlight and heat water in the center tube which is in a vacuum. The lack of pressure inside to tubes allows the boiling of water at a much lower temperature. Heat from the boiling water in the tubes is transferred to another liquid. This heated liquid travels through a house to a heat exchanger near/in the water tank. The heat exchanger again moves the heat out of the liquid and in to the water in the tank.

Flat Plate Collectors are sealed rectangular boxes with heat absorbing metal sheets next to small tubes of water or coolant. This liquid is moved through the house to the heat exchanger near/in the water tank. A schematic of this system can be found here.

How is a Solar Thermal System set up?

1.Size a system for house (depends on number of people and hot water needs) Generally, 80 gall tanks are sufficient for households with 3-4 people. Areas with less sunlight will need greater collector size. Sizing varies based on climate and hot water needs but a general rule of thumb according to Vermont's; Renewable Energy Resource Center is about 0.7ft2 and 0.85 ft2 of collector area per gallon of storage. Vermont RERC

2.Determine type of solar thermal system (flat plate vs. solar thermal...or another system!)

3. Determine whether you want an open loop system or a closed loop system. Open loop system have the benefit of less heat lost in transferring. Open loop systems can only use water as the liquid passing through the panels is the water coming out of your hot tap. Closed loop systems are not subject to mineral deposits from municipal water sources. Closed loop systems can use water or coolant.

3. Determine if you want to keep current water heater and purchase external heat exchanger or replace with tank that has an internal heat exchanger. If you are replacing your tank, it is often cheaper to by the internal heat exchanger (tank and exchanger are one unit) instead of purchasing two separate units that then have to be connected.

4. Choose to either install the system yourself or hire a contractor. Unless you have plumbing experience and are able to build these systems in accordance with local building codes, it would be advisable to hire a contractor to continue the job from here. The rest of these instructions will help you be an educated consumer...

5. Determine the optimum placement for the panels. This can be on your roof (ususally the best spot) or any other convenient location. In the northern hemisphere, south facing panels maximize sunlight exposure even during winter months when the sun is low in the sky. Be sure to maximize your solar window (the angular range of direct sunlight the panels receive during the day) and angle the panels equal to your latitude (40 degrees latitude-- 40 degrees angle towards south). Panels can be mounted on angled rooftops or on mounting racks for flat rooftops.Several appropedia pages such as Solar Radiation Maps and Solar Water Heating further discuss methods of determining the amount of light available for collection.

6. Select and purchase a hot water tank/heat exchanger system.

a. If you are keeping your old hot water tank you must purchase an external heat exchanger. The Piggyback system used in the spreadsheet is an external heat exchanger.

b. If you are purchasing a new hot water tank you can decide to have an internal or external heat exchanger. Internal heat exchangers come as a part of the water tank ( a single unit) external heat exchangers are sold separately from hot water tanks(exchanger and tank two separate units). Depending on your needs, you may choose one system over the other.

7.Determine if you require a freeze protection system such as a drain-back tank. If temperatures in a clients region drop below 40 degrees F, there is a potential for water to freeze in the system; This can cause extreme damage to a solar thermal system and create maintenance nightmares. To prevent water from freezing in the panels drain-back systems such as the one described below must be installed.

During night time when the solar panels are not receiving any light (and therefore heat), the water from the panels drains down in to a small tank inside the protection of the house. This prevents water from freezing in the pipes on your roof and potentially bursting any connections or cylinders. If you live in a climate that experiences any had frosts, a drain-back system is required. Several options for powering the drainback system are avaialable. A simple low flow water pump can be plugged in to the socket that turns off at night using a simple timer; or a small photovoltaic panel can power a pump(this system is nifty because during the nighttime when there is no sun hitting pv or thermal panel, pump shuts off and let water fall back in to drain-back tank-fully contained!!!).

8.Set up the piping. Try to find the shortest and easiest route through your house. A longer distance means more heat lost in transit and higher material costs.

9. Determine the heat transferring liquid.

a. If an open loop system has been selected, the heat transfer liquid must be municipal water.

b.If a closed loop system has been selected, the heat transfer liquid can be water or coolant. Coolant costs more than water but will not leave any mineral deposits in the system. Distilled water can actually leech minerals from the system and is not preferred.

10.Connect the pieces and let'er rip! The panels should start working when they first receive light.

## Problem statement

The final goal is to compare these two technologies for their ability to heat water for residential needs. The main questions we hoped to answer are;

-Can these systems supply all hot water needs? If not what percentage can they supply? Embedded in this question is, how much energy will a consumer save by installing these systems?

-Which system will cost the consumer less upfront and over time?

-Which system will last longer?

-Most importantly, what are the variables that make each of these systems an economical and environmentally friendly choice for consumers?

This spreadsheet will compare variable types of solar thermal collectors (flat plate and evacuated tubes) to air source heat pumps for heating domestic water needs. The client will enter in the following information:

•  Latitude of Clients location, # of sunny days, # of partly cloudy days, # of cloudy days

• Household data

o Number of people per house

o Appliances; dishwasher and laundry machine use frequency
o Showering/Bathing time
o Desired temperature for water tank
o Current hot water tank; cost, efficiency, life cycle, energy factor (for comparison to new systems)
• A solar heat pump model to compare
o Solar collector model/manufacturer
o Heat Transfer/water tank model/manufacturer
• A Heat Pump model to compare
• Desired Life cycle analysis length in years

Upon entering these specific variables for the client, information regarding each system, assumed water use, climate data and system components will appear in the spreadsheet. The assumptions and conversions used can be found to the right of the spreadsheet on the main page. Several tables with the data exist on subsequent sheets and can be used to add more options to the spreadsheet.
Based on data we found in our research and the user entered data the spreadsheet will return results comparing the following points:

The Bottom Line output

Annual energy costs to consumer after installing a solar thermal system or a heat pump system

Amount of money saved per year compared to using a conventional waterheating system

Payback time in years for a solar thermal system or a heat pump system

CO2 emissions in tons per year for water heating needs with a new system installed

Mercury emissions in grams per year for water heating needs with a new system installed

Life cycle cost over a yearly length

Additional outputs will show up on the spreadsheet. All variable outputs have been embedded in to the Bottom Line Output.

## Instructions

These are the instructions for using the spreadsheet. **It may be helpful to print these instructions to view next to the spreadsheet.**

1.Select the latitude of clients location.(-90 to 90) 2.Enter the number of clear days per year, Enter the number of partially cloudy days per year, and enter the number of cloudy days per year. These three numbers should total 365 days. A note beneath the boxes will note when the total is 365. 3. Select the number of people in the household.(1-10) 4. Select yes/no for a dishwasher. 5. Select yes/no for a laundry machine. If yes, go to 6, If no go to 7. 6. Enter number of laundry loads per person per week. 7. Select yes/no for taking baths. 8. Select yes/no for taking showers shorter than 5 minutes. If yes, skip to 8, if no go to 7. 9. Enter average time of shower. 10. Enter the average water temperature(F) coming out of municipal water pipes. 11. Enter preferred temperature(F) for water heater. 12. Select the desired type of new water heater.** 13. Select a type of solar thermal system to compare. 14. Select a model of solar thermal system to compare. 15. Select yes/no for purchasing a new water heater tank. 16. Select a life cycle length to analyze the systems. 17. Compare the annual price per system, annual emissions, and payback time for a solar thermal system or an air source heat pump!

• The trendsetter contender is a heat exchanger and water tank combo. It is an automatic yes to be purchasing a new water tank. The cost of heat exchanger is calculated in to the cost of tredsetter contender.

## Justification of assumptions

This section is a reference, thorough justification of your assumptions and values. Use references you gained during your literature review to back these up.

 Assumptions Values Source Average milligrams of Hg/kWh 0.012 Energy star pdf CFL's and mercury table 1Energy Star Average lbs of CO2/ kWh(coal) 2.117 Dept. of Energy 1999 CO2 emissions by energy type EIA CO2 report Average lbs of CO2/ kWh(petroleum) 1.915 Dept. of Energy 1999 CO2 emissions by energy type EIA CO2 Report Average lbs of CO2/ kWh(natural gas) 1.314 Dept. of Energy 1999 CO2 emissions by energy type EIA CO2 Report U.S. dollars/ kWh 0.12 U.S Energy Information Administration State by State Breakdown U.S. dollars/therm 0.98 Adams PG&E bill from Arcata California (two person home) Maximum solar Thermal efficiency 70% natural gas water heater efficiency 65% ACEEE] gallons used per faucet minute 3 common faucet maximum flow rate gallons used per dishwasher load 8 Average for old dishwashers pre1994,non-energy star Energy Star Q&A gallons used for bath 40 volume of 1/2 full short length bath or 1/4 of full length bath (L*W**H*7.5= ft3 to volume in gallons) gallons used per shower minute 2.5 Energy Star Q&A] gallons used per laundry load 40 Low efficiency:40 high efficiency:28 EPA Water Challenge

## Results

### Life Cycle Cost

A standard water heater lasts about 10-15 years. A solar thermal system however can last up to 30 years. We used a 15 year total life cycle cost of unit cost, installation, maintenance costs, and annual energy costs to find these results. These results are based on household size and how much water the people in that house are using. To attain this data we preset the water usage to what we considered a "moderate water usage" and tracked that data for each household size and then to a "conservative water usage" to track that to different household size. "Moderate water usage" mode assumes that the household has a dishwasher, a laundry machine, and that each family member does 1 load of laundry per week. It also assumes that each person takes 10 minute showers. Conserving mode assumes that the household does not have or does not use a dishwasher or laundry machine, and that each person in the household takes 5 minute showers. The different levels of cost for each system are then determined by how many people are in the household. This graph is also preset to some other variables. For example, it is assumed that a high efficiency gas water heater is used for a backup on the solar thermal. It is also assumed that the user lives in the Humboldt area as the graph was set to this areas climate specifications.

### Annual Energy Savings and Buy Back Period

This Chart shows us, based again on a moderate water usage, how long it will take for a system to buy itself back and contrasts that with the annual energy savings. The Annual energy savings assume that the water heater currently installed is a standard electric water heater.

### CO2 Emissions

The CO2 analysis is based on the conserving mode of hot water usage. We chose to set the table to this mode because we assumed that if a household is concerned with CO2 that they will also try to conserve energy as much as possible. Also, at any mode it was determined that solar thermal will use less CO2 because most of it's energy is solar and thus carbon neutral.

### 30 year Life-cycle cost

A solar thermal system typically lasts about 30 years. This chart shows what each system will cost over that period of time depending on daily hot water usage. To find your daily hot water usage fill out the spreadsheet provided under references. As you can see at about 25 gallons the lines cross and solar thermal becomes more cost effective over its 30 year life-cycle. In the 30 year life of a solar thermal system, because water heaters typically only last 10-15 years, the heat pump water heater or the water heater you had originally installed for your solar thermal system would need to be replaced once or twice. The cost of this replacement is all factored into the life-cycle cost of this chart.

### Average Total Cost Over Time

The average household uses about 60 gallons of water per day. This Chart is based off a 60 gallon a day hot water usage with 60 gallon peak hour demand. Like the other charts in our results it also assumes a 58 degree ground water temperature and all of the other climate specifications of Arcata, CA. As you can see, the cost at first may be cheaper on average with a heat pump water heater, but after about 20 years, after the heat pump has been replaced once, the total amount spent on the solar thermal drops below the total cost of the heat pump.

## Conclusion

The answer to our question, Which type of system should we suggest to a client? Our favorite answer: It depends! Certain conclusion can be found below; These are based on Humboldt County sunlight values and all of our assumptions. Please see the assumptions table earlier in this page and the assumptions table in the excel spreadsheet.

If you use less than 25 gallons of hot water per day, it is cheaper over a 30 year period to use air source heat pumps. This means you will need to replace the heat pump once (lifespan off 15 years so 30 years needs two heat pumps).If you use more than 25 gallons of hot water per day, it is cheaper over a 30 year period to purchase and install a solar thermal system. The lifespan of this system is about 30 years but costs about twice as much upfront.

This is a very broad based conclusion but through our data it seems like air source water heaters would be more applicable in the settings of a single apartment with limited space and smaller water use. The solar thermal system would be more applicable in a four or more person house that uses more water and has more space for the system. It seems that for households in-between these two extremes it would be best to fill out the excel spread sheet we created so that the needs of the client can be better quantified.

Broad conclusion: For low water use households such as single person dwellings or water conscious users, heat pumps appear to be a very smart decision. However, high occupancy homes and high water use needs would be most positively impacted by solar thermal systems.

## Discussion

Heat Pump

The GE air source heat pump was chosen for this project because it had a high energy factor and for its easy installation. The installation of the GE air source heat pump is exactly like a regular water heater and costs about $400 dollars for professional installation. The GE unit switches automatically from four different settings. For most of the time it will be in "eheat" setting which means that only the air source heat pump is working. When a the water usage goes above the peak demand of 63 gallons it goes into hybrid mode which is the air source heat pump and the standard hot water heating component working as one to keep up with demand. There is a standard electric mode in which it works like a regular electric hot water heater and a high demand mode for households with higher than average water use. In this mode the electric hot water heater is working very hard.  We decided to stay away from air source hot water heater add on units. These units add on to existing electric water heaters. We decided to stay away from the add on units after a talk with Maples Plumbing in Eureka. According to Maples Plumbing it doesn't make sense to buy these units because although upfront they are about half the price of the combined air source hot water heater unit, they have a high cost in installation. These attachable air source heat pumps also have a lifespan of 20 years, and the tanks on the electric water heaters have a life span of about 10 to 15 years. So the limiting factor is the electric hot water heater tank. Unless a person just installed a new electric hot water heater this technology wouldn't be practical according to Maples Plumbing. It is important to note in the final cost of the air source heat pump we did include a tax rebate from the Federal government. this rebate comes back to the buyer when they fill out their federal a tax form. Under the current tax rebate a person can obtain 30% of the cost of the unit and installation up to$1500 dollars.

 Benefits of 'Air Source Water Heater' Downsides to 'Air Source Water Heater' Lifespan as long as regular water heater (15 yrs) Air temp must be above 40 degrees F for the heat pump unit to work Installation is like regular electric hot water heater Less efficient under high constant demand for water Little maintenance (filters changed once a month) Needs to be in an enclosed area (cant be outside) Federal tax rebate of 30% of cost of unit and installation up to $1500 The room must be at least 10' x 10' x 7' or larger (because it draws in surrounding air) Reduces carbon footprint Solar Thermal Solar thermal panels work extremely well in very sunny climates and less so in cloudy climates. Systems capturing lots of light can supply most if not all of your hot water needs. Continuous cloudy days will cause your back up heater to be working more. There is a point where the solar thermal system doesn't get enough sunlight every day to heat very much of your water. This means that the cost of heating your water (using electric, gas or other source) will be inversely proportionate to the amount of sunlight you get. More sunlight= back up system working less, less sunlight=back up system working more. In the fog belt region of Humboldt County, CA, solar thermal panels have a long buyback time (years it takes for water heating savings to accumulate to pay off entire cost of system). For other areas in Humboldt county such as the more sunny mountainous regions (especially during summer months, winter months not so much) solar thermal system could play a larger role in water heating needs. Solar Thermal systems are ideal for heavy hot water loads. Multiple person houses would do well to choose solar thermal systems over air source heat pumps. Federal tax rebates are currently available for solar thermal systems. There are several qualifications that must be met in order to get the rebate. The requirements are as follows; at least half of all energy used in the dwelling must come from solar, the system installed must be SRCC certified and only costs on the solar thermal system are eligible for rebates. If your residence meets these requirements, 30% of the entire cost of the system can be refunded on your next federal tax bill. If the 30% rebate is greater than what you pay in federal taxes in the year of installation, 100% of your federal taxes are refunded to the purchaser.  Benefits of Solar Thermal System Downsides to Solar Thermal System Long lifespan of system-30years Expensive start up costs-approx.$4000-\$5500 Easily expandable-more panels are easy to place in series Long buyback time in cloudier climates- about 20 years Efficient for high water use Difficult to install without prior experience No maintenance-system is self-sufficient (assuming no damage) Much more complicated system than conventional water heaters Energy self-reliance- reduces costs of heating Doesn't provide 100% of water needs at high demand volumes or low sunlight

This section is a list of several future steps to improve the accuracy and adaptability of the spreadsheet. Some of these suggestions are slight alternations to variables currently in the spreadsheet while some ideas are completely new.

-The values for sunlight per latitude do not take in to account the seasonal changes. In areas that get very little sunlight during winter months (winter in northern hemisphere) the back up system will be working the majority of the time. During summer months in high latitude areas, continuous sunlight for many hours every day will heat water needs well above what the household will use. The total amount of sunlight hitting the panels does not directly equate to how much water heating costs will be taking in to account seasonal variation.There needs to be a way to equate the seasonal differences. In extremely cold climates, these systems become inappropriate due to low collection rates and excessive costs using a back up system. There is a threshold around 60 degrees latitude where the winter months have too little sunlight for thermal systems to provide any input of heat. Any system not being used for portions of the year might not be the most economical solution.

-Pollution from system manufacture: We did not examine the materials and production externalities from these two systems. The heat pump requires replacement every 15 years where as solar thermal replaced every 30 years. Don't know pollution externalities, recycling potential of materials

-Ground source heat pumps were not examined in this project. They are another type of heat pump that is commonly used for hot water heating. High installation costs with this system

-Climate variation could be a little more concrete- it was difficult to find consistent sunlight values for regions, we just used NOAA national weather service values, they seemed most consistent

-Other types of hot water heaters could be included: On-Demand water heaters could be coupled with either of the examined systems

-Cost outputs could incorporate federal and state tax rebates.

## References

sizing for a solar system http://www.fsec.ucf.edu/en/consumer/solar_hot_water/pools/sizing.htm

California renewables portfolio http://web.archive.org/web/20160102050109/http://www.cpuc.ca.gov:80/PUC/energy/Renewables/index.htm

EIA energy outlook for 2009 http://www.eia.doe.gov/oiaf/ieo/world.html

Solar thermal energy applications https://www.appropedia.org/Solar_thermal_energy_%28original%29

Heat-Transfer Fluids for Solar Water Heating Systems http://web.archive.org/web/20120702213242/http://www.energysavers.gov:80/your_home/water_heating/index.cfm/mytopic=12940

Solar Water Heater Energy Efficiency http://web.archive.org/web/20120814093604/http://www.energysavers.gov:80/your_home/water_heating/index.cfm/mytopic=12900

Evaluating Your Site's Solar Resource for Solar Water Heating http://web.archive.org/web/20120822023447/http://www.energysavers.gov:80/your_home/water_heating/index.cfm/mytopic=12870

Siting Your Solar Water Heating System's Collector http://web.archive.org/web/20120823032408/http://www.energysavers.gov:80/your_home/water_heating/index.cfm/mytopic=12890

Notes from class presentation on solar thermal heating https://www.appropedia.org/Solar_Thermal_Panels

Solar Water Heating System Freeze Protection http://web.archive.org/web/20120815110611/http://www.energysavers.gov:80/your_home/water_heating/index.cfm/mytopic=12960

southface.org

Heat pumps: Michael Winkler, here at RCEA: 7 >>zero << 7-269-1>>seven<<00 Solar thermal: Ben Scurfield, Scurfield Solar: 4>>four<<3-07>>five<<9 http://web.archive.org/web/20190119081032/https://www.scurfieldsolar.com/ Alchemy Construction: http://www.alchemyinc.com/ Solar H2OT: http://web.archive.org/web/20191002001339/http://www.solarhotwaterplus.com:80/index.htm Newsflash: we're hosting a workshop tomorrow night by Solar H2OT on solar hot water systems, where you can get an introduction to one approach by a local installer. Here's a link to the announcement: http://www.redwoodenergy.org/EventsDetail.asp?EventDate=10/14/2009&EventID=222&Rec=1&DateID=294 Dana Boudreau Operations Manager Redwood Coast Energy Authority 707.2>>six<<9.17>>zero<<0 www.redwoodenergy.org

Page data
Type Project heat pump, solar, solar thermal, solar heating SDG07 Affordable and clean energy, SDG11 Sustainable cities and communities adam, Devan Hemmings, Robert Duncan, Ryan 2009 CC-BY-SA-3.0 Cal Poly Humboldt English (en) 7,277 adam, Devan Hemmings, Robert Duncan, Ryan (2009). "RCEA Solar thermal vs Heat pump". Appropedia. Retrieved August 9, 2022.
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