Our company has received many inquiries from homeowners and business owners interested in learning more about solar thermal systems. We know they are interested in the types of solar thermal collectors, storage tanks and heat exchangers that make up the system, but their first 2 questions to us are: How much does it cost and have much will I save each in year in fuel costs? If it's not affordable to install the system then it's really not necessary to know what components make up the system. So let's address the cost question first.

Homeowners and business owners have many options to choose from when it comes to solar thermal heating, including:

1. Propylene Glycol Active Indirect System
2. Drainback Active Indirect Systems
3. Integral Collector Storage (ICS) Passive Direct System
4. Thermosiphon Passive Direct System
5. Thermosiphon Passive Indirect System
6. Controller-Based Active Direct Systems

1. Glazed Flat Plate Collector
2. Evacuated Tube Collector:
3. Unglazed Flat Plate Collector
4. Integrated Collector Storage (ICS)

Do you want your solar thermal system to heat domestic hot water only or also to heat your entire home or business. Domestic hot water is used for bathing, laundry and dishwashing.

Installing a solar thermal system to heat domestic hot water is cost effective since hot water for bathing, laundry and dishwashing is required year round. Installing a solar thermal system to heat your home or building is expensive, but it will work. Less expensive renewable energy options for heating homes and buildings include using a geothermal heat pump with some assist from a solar thermal system. Refer to the geothermal section of this website for more information.

System Description of Most Commonly Used Solar Thermal System in New England

To keep it simple and affordable, let's limit our discussion to the most commonly used solar thermal hot water heating system used in New England, which is using: three glazed flat panel solar collectors, an 80-gallon hot water storage tank and a heat exchanger configured in a closed loop, propylene glycol active indirect system for heating domestic hot water.

This system and its various components are discussed in more detail in the "Additional Information" section below, but basically this is a closed loop system that is separate from the potable (drinkable) hot water system. A non-toxic fluid (propylene glycol) is circulated through the solar thermal collectors when the temperature of the collectors is a few degrees warmer than the temperature of the water in the hot water storage tank. Temperature sensors located in the solar thermal collectors and the hot water storage tank determine when to start and stop the fluid from circulating through the collectors. A very low wattage pump is needed to circulate the fluid. In fact, the pump is so small that a small photovoltaic panel is often used to provide power to the pump.

The fluid is heated as it circulates through the solar thermal collectors. The heated fluid then leaves the collectors and flows through a heat exchanger located in the 120-gallon hot water storage tank. The heat exchanger transfers the heat from the fluid to the water in the storage tank. The fluid then cools as it leaves the hot water storage tank and heads back up to the solar thermal collectors to be heated again. This process continues as long as the fluid in the solar collectors is warmer than the water in the hot water storage tank.

System Description of Most Efficient Solar Thermal System Used in New England

The circulating fluid used in the system describe above is a 50-50 mixture of distilled water and propylene glycol. Distilled water is very efficient at collecting heat available as it circulates through the solar thermal collectors. Adding propylene glycol reduces the system efficiency to about 70%, which means 70% of the heat collected in the solar thermal collectors is transferred to the water in the hot water storage tank. Propylene glycol is added to water to prevent the water from freezing.

A more efficient, but riskier, way to collect heat is to use 100% distilled water as the circulating fluid. This type of system is called a Drainback Active Indirect System. All water in the collector and pipes drains from the system into a reservoir tank when the circulating pump shuts off. The system is suited to most climates. These systems work best in warm climates because they also drainback when the system has met a maximum set temperature in the storage tank. This automatic safeguard against overheating avoids problems when hot water demand is very low, such as during vacations. These systems must be installed so that water will completely drain from the collector and pipes when needed. These systems work in cold climates because they drain when the pump is not operating so that the circulating fluid is protected inside conditioned spaces.

There are several advantages and 2 big disadvantages in using a drainback system.

Drainback System Advantages

1. The system is more efficient.
   
2. The system requires less maintenance. Propylene glycol fluid needs to be tested periodically and replaced when needed.
3. Summertime maintenance of the drainback system is much easier since all the fluid drains from the system when there is sufficient hot water. In the propylene glycol system, the fluid is always present in the system, even when no additional hot water is needed. Therefore extra precautions need to be taken to ensure the propylene glycol fluid does not become overheated. Overheating this fluid will reduce its effectiveness requiring the fluid to be replaced.

Drainback System Disadvantages

1. The system requires a larger pump motor to pump up the fluid from the reservoir tank to the collectors each morning. It's possible to minimize the size of the pump motor by locating the drainback reservoir tank as close to the collectors as possible. The propylene glycol system requires a very small pump motor since it just circulates the fluid. There is no need to pump up the fluid to the collectors since the fluid is already there.
2. The system needs to be perfectly installed to ensure all water can drain from the system each afternoon, especially when temperatures are below freezing. If water freezes in the solar thermal system, major damage (and cost) could occur.

Estimating Solar Thermal System Size Needed

Estimating how many solar thermal collectors you'll need to meet at least one-half of your domestic hot water requirements is not easily done by hand. It requires knowing ground water temperatures and performance specifications of the solar thermal collectors you're considering. Several solar thermal calculation programs have been developed to make this estimation procedure easier. We use the solar thermal calculation program available at ww.RETScreen.net. This calculation program may be downloaded for free after completing a registered user form. The program was developed as a collaborative effort by several different organizations, including the Natural Resources Canada and NASA.

Besides the solar radiation data and the ground water temperature data for every region in the world, the really nice feature about RET Screen is that it includes the performance specifications for just about every solar collector that has been tested and certified by the SRCC (Solar Rating & Certification Corporation, www.solar-rating.org).

It should be noted at this point that in order to qualify for the Federal Solar Tax Credit, the solar collectors must have an SRCC rating and the solar thermal system must be sized to meet at least one-half of your domestic hot water requirements. State solar incentive programs have varying requirements so please check with your state's solar incentive program coordinator before investing in a solar thermal system. Listed below is general information about various state solar incentive programs.

Estimating Your Energy Usage

To estimate your energy usage, let's first assume your hot water tank is set to 125 ⁰F. Let's next assume we're estimating the domestic hot water needs for a 4-person household and each person uses 33 gallons of water per day. Therefore the daily hot water requirement is 132 gallons of hot water per day.

The amount of energy expended to heat that water depends on your starting temperature and ending water temperatures. We've already said the ending water temperature is 125⁰F. The starting temperature is the temperature of the water that enters your hot water tank. If you live in Caribou, Maine, which is the northern most town in Maine, the ground water temperature at the coldest time of year is 34⁰F. It takes about 8.3 Btu to increase 1 gallon of water by 1⁰F. Which means it takes about 755 Btu's to raise the temperature of 1 gallon of ground water from 34⁰F to 125⁰F. In the summertime, the ground water temperature in Caribou increases to 50⁰F. Therefore it takes only 623 Btu's to raise the temperature of 1 gallon of ground water from 50⁰F to 125⁰F.

The next item to consider is how much solar radiation is available in Caribou. According to RET Screen, average solar radiation available to be absorbed by solar thermal collectors installed in Caribou ranges from a November low of 2.66 kWh per square meter of collector area per day to a July high of 4.93 kWh per square meter of collector area per day. Refer to Table 1 below to view this data from RET Screen.

The last major item to consider in estimating the solar thermal system size is the ability of the solar thermal collectors to absorb the solar radiation available. According to the RET Screen Analysis shown in Table 2 below, a solar thermal system consisting of (3) Heliodyne Gobi 408 glazed flat plate collectors and (1) 120-gallon hot water storage tank with heat exchanger, will produce 18.43 million Btu per year in Caribou, Maine (this figure is shown on the last line of Table 2). A typical 4 person home in Caribou, Maine will use 132 gallons of hot water per day and require 33.84 million Btu per year to heat that water (this figure is shown on the last line of Table 1).

Once we've figured out the ground water temperatures, the amount of daily solar radiation available and the type and size of the solar thermal system to be installed, we can now calculation the cost and payback of the system. Let's look at two extreme locations in New England and one centrally located New England city to compare energy requirements and solar thermal system performance.

RET Screen Solar Resource & Heating Load Calculation

Tables 1, 4 and 7 below provide solar radiation and ground water temperature data from the RET Screen solar thermal calculation program for Caribou, Maine, Concord, NH and New York City, respectively. The only inputs required from the user are:

A. Slope of collectors.

1. For Domestic Hot Water Purposes: To maximize the average yearly collector output for domestic hot water purposes, the slope of the collectors should be equal to the latitude of its location. For example, Caribou, Maine has a latitude of 46.9⁰. Therefore, the collectors should be positioned at an angle of about 47⁰ from the horizontal plane.
2. For Space Heating Purposes: The greatest need for space heating occurs during the winter when the sun is at its lowest angle. Therefore, to maximize solar heat gain when the sun is at its lowest angle, the slope of the collectors should be equal to the latitude of its location+15⁰. For example, Caribou, Maine has a latitude of 46.9⁰. Therefore, the collectors should be positioned at an angle equal to latitude +15⁰ or an angle of about 62⁰ (47⁰ + 15⁰) from the horizontal plane.

B. Daily hot water usage in liters. To convert gallons into liters, multiply gallons by 3.785 to obtain liters. Refer to Table 10 below for estimated hot water usage based on number of persons living in the home.

C. Temperature of hot water in ⁰C. Enter the temperature setting on your hot water tank in ⁰C. If your tank is set at 125⁰F, then enter 52⁰C. If your tank is set at 130⁰F, then enter 54.4⁰C.

Once you've completed the above 3 steps, RET Screen determines the amount of million Btu's required each year to meet your hot water requirements. For example, RET Screen has determined that a 4 person household in Caribou, Maine, using 500 liters (132 gallons) of hot water per day, requires 33.91 million Btu per year of energy to meet those hot water requirements. You can see this value on the very last line of Table 1, which is listed as "Energy demand for months analyzed."

RET Screen Energy Model

RET Screen will now analyze the performance of your solar thermal system based on the type and quantity of solar thermal collectors you chose and the amount of hot water storage capacity is available. A good rule of thumb is that an 80 gallon hot water storage tank is needed for every 2 glazed, flat solar collectors installed (each collector size is about 4 feet wide by 8 feet high).

Tables 2, 5 and 8 below show the final RET Screen analysis if 3 Heliodyne Gobi 408 collectors and a minimum of 100 gallons (381 liters) of hot water storage is installed in Caribou, Maine, Concord, NH and New York City, respectively. Each of the analysis is based on a 4 person household using 500 liters (132 gallons) of hot water per day. The estimated renewable energy delivered by a chosen solar thermal system with an efficiency of 70% can be seen in the last line of Tables 2, 5 and 8 and is identified as "Renewable energy delivered." All 3 tables indicated a minimum of 50% of each households hot water energy requirements will be met by the solar thermal system.

Estimating Your Cost and Energy Savings

Tables 3, 6 and 9 below show the final RET Screen analysis if 3 Heliodyne Gobi 408 collectors and a minimum of 100 gallons (381 liters) of hot water storage is installed in Caribou, Maine, Concord, NH and New York City, respectively.

Table 1: Caribou, Maine - RET Screen Solar Radiation Analysis for 4-Person Home (22 kb PDF)

Table 2: Caribou, Maine - RET Screen Analysis of Solar Thermal System for 4-Person Home (22 kb PDF)

Table 3: Caribou, Maine - Solar Thermal Cost & Performance Analysis for 4-Person Home Using 132 Gallons (500 Liters) of Hot Water Per Day (19 Kb PDF)

Table 4: Concord, New Hampshire - RET Screen Solar Radiation Analysis for 4-Person Home (22 Kb PDF)

Table 5: Concord, New Hampshire - RET Screen Analysis of Solar Thermal System for 4-Person Home (20 Kb)

Table 6: Concord, New Hampshire - Solar Thermal Cost & Performance Analysis for 4-Person Home Using 132 Gallons (500 Liters) of Hot Water per Day (19 Kb PDF)

Table 7: New York City, New York - RET Screen Solar Radiation Analysis for 4-Person Home (19 Kb PDF)

Table 8: New York City, New York - RET Screen Analysis of Solar Thermal System for 4-Person Home (20 Kb)

Table 9: New York City, New York - Solar Thermal Cost & Performance Analysis for 4-Person Home Using 132 Gallons (500 Liters) of Hot Water Per Day (19 Kb)

Table 10: Number of Solar Thermal Collectors Required to Qualify for $2,000 Federal Solar Tax Credit Available to Homeowners (17 Kb PDF)

Solar Incentives: Rebates and Tax Credits

The following is a list of federal and state rebates and tax credits offered to homeowners and business owners who install solar thermal systems at their location. To qualify for the rebate and tax credits, the solar thermal collectors must have a SRCC certification rating of OG-100. A complete solar thermal system that that has met the SRCC certification requirements will have a rating of OG-300. Either one of these ratings meets the requirements for the Federal Solar Tax Credit.

In addition to the SRCC certification ratings, the states offering solar thermal rebates or tax credits require the system to be installed by a certified solar thermal system installer. Check with applicable state programs for specific requirements, including a list of certified solar thermal system installers. Homeowners and business owners who install the solar thermal system themselves will be not eligible for the tax credits and state rebates if the installer is not a licensed plumber or a certified solar thermal system installer.

If you're interested in the rebates offered in a state not listed below, send us and email This e-mail address is being protected from spam bots, you need JavaScript enabled to view it . We'll check into the PV rebates offered by the state and update our website to show that information. Also, if you have information to share with us about your state's solar thermal rebate program and your fuel costs, please send us an email. We'll include your information on our website. For the latest information concerning federal and state rebates and incentives for all states, please visit www.dsireusa.org.

Federal Tax Credits for Homeowners

Federal Tax Credits for Business owners

Connecticut Solar Thermal Rebate Program

Maine Solar Thermal Rebate Program

Massachusetts Solar Thermal Rebate Program

New Hampshire Solar Thermal Rebate Program

New York Solar Thermal Rebate Program

Rhode Island Solar Thermal Rebate Program

Vermont Solar Thermal Rebate Program

List of local Solar Thermal Installers

Federal Tax Credits for Homeowners:

Homeowners may deduct 30% of the cost to install their solar thermal system, up to $2,000, from their federal taxes.

For example, a New England homeowner who installs an $10,000 solar thermal system may claim a tax credit of $2,000 (30% of $10,000 = $3,000, which exceeds the $2,000 limit). The total cost of the solar Thermal system is now $8,000.

Homeowners who receive state rebates for installing a solar thermal system will not be able to claim the $2,000 federal tax credit, unless the homeowners rebate from the state is less than $2,000. If this is the case, the homeowner could claim the difference on their federal tax returns.

Federal Tax Credits for Business owners:

Business owners may deduct up to 30% of the cost of the solar thermal system from their federal taxes. There is no maximum amount they claim on the tax forms. In addition, business owners may also depreciated the cost of the system in 5 years. The typical payback is 8 years for a business owner who is able to take full advantage of the 30% tax credits and the 5 year accelerated depreciation tax credit.

Table 11: Example of Federal Solar Business Tax Credits Available For Laundromat Located in Dover, New Hampshire (13 Kb PDF)

Business owners who receive state rebates for installing a solar thermal system will not be able to claim the federal tax credit, unless the business owners rebate from the state is less than the 30% federal tax credit. If this is the case, the business owner could claim the difference on their federal tax returns. The business owner may still take advantage of the 5 year accelerated depreciation tax credit.

Back to Top

Connecticut Solar Thermal Rebate Program:

None. Refer to the Federal Tax Credit programs detailed above.

Connecticut offers sales tax and property tax exemptions for solar thermal systems. Refer to the following link for more information:

Information for this program is available at http://www.ctinnovations.com/funding/ccef/solar_rebates.php

Maine Solar Thermal Rebate Program:

Maine does have a solar thermal rebate program in effect for years 2006 and 2007. The program provides rebates to home and business owners, including non-profit organizations, for 25% of the cost of the system, up to a maximum of $1,250. Information for this program is available at http://www.maine.gov/msep/programs_solar.htm.

Home and business owners may also qualify for the Federal Tax Credit programs detailed above.

Back to Top

Massachusetts Solar Thermal Rebate Program:

Residents

Massachusetts residents may claim a 15% credit against the state income tax for the cost of a solar thermal system (including installation) installed on an individual's primary residence. The maximum credit is $1,000, and the credit may be carried over if the credit is greater than income tax liability in one year. For more information, please visit http://www.state.ma.us/doer/programs/renew/renew.htm#taxcred

Businesses

Businesses may take a credit against the state excise tax for the purchase and installation of a solar water-heating system. The system must be installed at a commercial building, and the credit may not exceed $300 or 15% of the system cost, whichever is less. This credit expires December 31, 2007. A business may carry over unused portions of the credit for use in taxable year 2007.

Note that a credit for the installation of a solar water heating system in a commercial building between November 1, 2005 and March 31, 2006 may be applied for the taxable year 2006.

A business that takes this credit may not also take the state tax deduction allowed under MGL ch. 63, § 38H for the same solar water-heating system.

Businesses & Homeowners

Massachusetts home and business owners may qualify for property and sales tax exemptions. For more information, please visit http://www.state.ma.us/doer/programs/renew/renew.htm#taxcred

KeySpan offers rebates to its residential, commercial, industrial, and multifamily customers for the installation of solar thermal technologies. KeySpan first conducts a free energy audit to interested participants to identify appropriate solar thermal technologies as well as estimated natural gas savings. Recommended solar thermal applications may include solar hot water heating, solar space heating, solar pool heating, and/or high temperature process applications.

Residential customers receive a $1,500 rebate for solar water heating systems. Commercial, industrial, and multifamily customers receive a one-time rebate of $1.50 per therm of estimated first-year savings, up to 50% of the project costs or $100,000 per project. For more information, refer to: http://www.keyspanenergy.com/psbusiness/energy/energy_efficiency_ma_kedma.jsp

Home and business owners may also qualify for the Federal Tax Credit programs detailed above.

Back to Top

New Hampshire Solar Thermal Rebate Program:

None. Refer to the Federal Tax Credit programs detailed above.

Back to Top

New York Solar Thermal Rebate Program:

Residential solar thermal systems are typically eligible for a New York State income tax credit for 25% of the cost of the system, up to a maximum of $5,000. Please visit the NYS Department of Taxation and Finance for more information.

Back to Top

Rhode Island Solar Thermal Rebate Program:

Rhode Island offers a personal tax credit for solar thermal systems. The tax credit is equal to 25% of the system cost and applies to residential installations. solar thermal systems must have a minimum solar collector area of 60 square feet. Systems up to $7,000 are eligible for the full 25% credit. (Owners of systems that exceed $7,000 in cost will receive a credit based on a $7,000 system cost.)

Like the property tax provisions of many other states, Rhode Island law specifies that for purposes of local municipal property tax assessment, certain solar thermal systems cannot be assessed at more than the value of a conventional heating system, a conventional hot-water system or energy production capacity that otherwise could be necessary to install in a building.

Solar thermal systems and equipment sold in Rhode Island are exempt from the state's sales and use tax.

For more information, please visit http://www.energy.ri.gov/programs/renewable.php

None listed for business owners. Refer to the Federal Tax Credit programs detailed above.

Back to Top

Vermont Solar Thermal Rebate Program:

Solar hot water systems must be complete systems capable of serving domestic hot water loads. The thermal collectors used in the systems must have an OG-100 output rating from the Solar Rating Certification Corporation (SRCC) or an equivalent organization. Upgrades to existing solar hot water systems are also eligible, but to be eligible, an upgrade must include new collectors with a rated output of at least 15 kBtu/day. Systems that heat water for swimming pool use are not eligible.

For multi-family low-income projects, program incentives will support the 1,000 kBtu/day rating for solar hot water collectors; larger systems are eligible but will receive an incentive capped at the lesser of $35,000 or 50% of the total installed cost of the system.

Incentive Amount ($/100 Btu/day) by Customer Type:
Individuals, businesses - $1.75
Multi-family low-income housing projects - $3.50
Maximum Incentive:
Individuals, businesses--$8,750
Multi-family low-income housing projects - The lesser of $35,000 or 50% of total installed cost

Solar hot water system incentives are calculated according to the Solar Rating and Certification Corporation's OG 100 collector rating for kBtu/Day output under Clear C conditions. Alternative equivalent ratings for non-flat plate collectors may be submitted for consideration.

For more information, please visit http://www.rerc-vt.org/incentives/index.htm

Business owners may also qualify for the PV rebate program. Overall cost and payback is identical to that listed under the Federal Tax Credits for Business owners section.

Back to Top

Additional Resources:

The following links provide additional information concerning solar thermal system installations.

The link below provides a detailed overview of the different solar thermal systems available and the installation procedures.

High-Performance Home Technologies: Solar Thermal & Photovoltaic systems; Volume 6, Chapter 3. NREL/TP-550-41085; PNNL-16362; June 2007 (1.5 Mb PDF)

Home Power magazine's online website, www.HomePower.com, provides an easy to understand tutorial about solar thermal energy basics. Lots of other great information is available at Home Power's website. Every member of our company has their own subscription to this magazine.

Solar Energy Industries Association "Guide to Federal Tax Incentives for Solar Energy, Version 1.1." Downloaded from www.seia.org (1 Mb PDF)

List of local Solar Thermal Installers:

Lee Consavage and David Plante of Seacoast Consulting Engineers have recently certified 3 solar thermal systems for homeowners in southern Maine. The following licensed master plumbers installed these systems and are highly recommended by the homeowner’s who hired them:

<!--[if !supportEmptyParas]-->

<!--[if !supportEmptyParas]-->

Back to Top