Is Solar Heating For You? Narrative Discussion

THE SOLAR RESOURCE – One factor that really helps with solar heating performance is the fact that the sunshine often comes when you really need it. In Vermont, November is a quite cloudy month, but it is generally mild. The cloudiness tends to result from warmer air coming up from the South. The brutally cold weather that we get in winter, along with much of the Northern parts of the US, is the result of very clear, cold and dry air coming down from the North. The result is the most brilliant sunshine that we get all year, just when it is coldest. This “happy confluence of solar circumstances” is just what we need to get the most from our solar heating systems.

In Vermont, November is quite cloudy, but it is generally mild.

SOLAR STORAGE – We all know that the sun doesn’t always shine during the day, and that it never shines at night. This means that we need a certain amount of storage potential for our solar energy in order for solar to meet a high percentage of our needs (a high solar heating fraction).

A solar heating system for use in a Vermont home needs about 2 times as many solar collectors and twice as much thermal storage as a home in Colorado would. This is not as much of a burden as it may seem. The cost of the controls and mechanicals will be the same, and if the cost of thermal storage is reasonable, things can work out fairly well. An alternative would be to construct a building that conserves energy better in the cloudier climate.

SOLAR HEATING FRACTION – The solar heating fraction is the percent of your energy need that solar energy will meet. It is a function of the same four items that are considered for a solar performance calculation, i.e.:

a. The Solar Resource.

b. The energy requirement to be met.

c. The ability to store solar energy for use in cloudy periods.

d. The efficiency of the system.

It is theoretically possible to achieve a 100% solar heating fraction in cold climates, and some purists will do what it takes to do accomplish this goal. In general however, the over design that is needed to meet peaking needs under low sun conditions results is a wasted investment for much of the year. It is neither necessary nor even desirable to reach the 100% in many cases. Extremes in architecture can be costly. It is never a good idea to spoil the architecture of a building with too many solar collectors. But, to each his own.

If a person needed $100 and found a $50 bill on the floor, most people would be wise enough to pick it up. If a solar energy system “only” provides 50% of the heating requirement, that could still be very nice. Such a system could provide all of the heat for much of the year and luxurious quantities of free domestic hot water and ventilation in sunny periods. It should provide a very nice tax-free return on the investment.

If you factor the four important criteria for solar performance these will be the results for various areas of the continental United States. We will assume the example of a 2,000 square ft building, and we will assume the use of solar collectors measuring 4’x8′. We also assume that the solar collectors will face within 20 degrees of True South and that the angle of the collectors will equal the latitude plus 15 degrees.

Scroll past the maps to estimate Solar Heating Fraction by Climate Zone of the Unites States.

Global Horizontal Solar Irradiance

This map provides annual average daily total solar resource using 1998-2016 data (PSM v3) covering 0.038-degree latitude by 0.038-degree longitude (nominally 4 km x 4 km). For more information, please visit NSRDB.

See monthly maps and the above map here.

Map from https://www.nrel.gov/

USDA Plant Hardiness Map

USDA plant hardiness map from the U.S. Department of Agriculture gives a sense of the climate but doesn’t really tell you how much sun that means.

Click here to visit the interactive version,where you can download state/territory maps or find regional and national maps for download.

Map from https://planthardiness.ars.usda.gov

CLIMATES WITH VERY COLD WINTERS AND SOME PROLONGED CLOUDY SPELLS

(The Northern Tier of the United States)

SOLAR FRACTION
100%	Architectural flexibility will be very restrictive. You must basically construct an underground home, or one that is very aggressively bermed with earth. This design will give very high efficiency and the very high heat storage within the ground that will be needed. (Solar Option One) You will need 8-10 solar collectors. Cost effectiveness will be good.
80-90%	Architectural flexibility will be moderate. Slab on grade construction will be needed in significant areas of the house. Solar energy can be stored within the slab and the compacted earth beneath. (Solar Option One) You will need 8-10 solar collectors. Cost effectiveness will be good.
Up to 50%	Excellent architectural flexibility. No particular measures to store heat within the structure of the house or the materials that the house is made from. Use a radiant heating system using the domestic hot water heater as the heat source. Then use solar energy to make domestic hot water. (Solar Option Two) Add one or two extra domestic hot water heaters for added heat storage. You can use 5-6 solar collectors. Additional solar collectors will have limited value because of limited heat storage ability. Cost effectiveness is good up to about 40-50%, but declines rapidly thereafter.

CLIMATES WITH SOMEWHAT MILDER WINTERS AND MORE ABUNDANT AND MORE CONSISTENT SUNSHINE

(Southern New England to Virginia, the Midwest, Northern California)

SOLAR FRACTION
100%	Architectural flexibility will be very restrictive. You must basically construct an underground home, or one that is very aggressively bermed with earth. This design will give very high efficiency and the very high heat storage within the ground that will be needed. (Solar Option One) You will need 7-9 solar collectors. Cost effectiveness will be good.
80-90%	Architectural flexibility will be moderate. Slab on grade construction will be needed in significant areas of the house. Solar energy can be stored within the slab and the compacted earth beneath. (Solar Option One) You will need 6-8 solar collectors and cost effectiveness will be good.
Up to 50%	Excellent architectural flexibility. No particular measures to store heat within the structure of the house or the materials that the house is made from. Use a radiant heating system using the domestic hot water heater as the heat source. Then use solar energy to make domestic hot water. (Solar Option Two) Add one or two extra domestic hot water heaters for added heat storage. You can use 4-5 solar collectors. Additional solar collectors will have limited value because of limited heat storage ability. Cost effectiveness is good up to about 40-50%, but declines rapidly thereafter.

THE SOUTHWEST (COLD WINTERS, GOOD CONSISTENT SUNSHINE)

(Colorado, Arizona and other States with similar climates)

SOLAR FRACTION
100%	Architectural flexibility will be somewhat restrictive. You must construct a home with considerable thermal mass and good to excellent energy conservation, or one that is very aggressively bermed with earth. Adobe or masonry is typical. This design will give very high-energy efficiency and the very high heat storage within the building and the ground beneath that will be needed. (Solar Option One) You will need 6-8 solar collectors. Cost effectiveness will be excellent.
80-90%	Architectural flexibility will be moderate. Slab on grade construction will be needed in some areas of the house. Solar energy can be stored within the slab and the compacted earth beneath. (Solar Option One) You will need 5-7 solar collectors and cost effectiveness will be excellent.
Up to 80%	Excellent architectural flexibility. No particular measures to store heat within the structure of the house or the materials that the house is made from. Use a radiant heating system using the domestic hot water heater as the heat source. Then use solar energy to make domestic hot water. (Solar Option Two) Add one or two extra domestic hot water heaters for added heat storage. You can use 5-6 solar collectors. Additional solar collectors will have limited value because of limited heat storage ability. Cost effectiveness is good up to about 60-70%, but declines thereafter.

CLIMATES WITH MILDER WINTERS AND GOOD SUNSHINE

(Florida, Louisiana, Texas, Southern California)

SOLAR
FRACTION

100%

Architectural flexibility will be good. You should construct a home of masonry materials that can store heat (or cool temperatures) and use slab on grade construction for added thermal mass. Use a radiant heating system using the domestic hot water heater as the heat source. Then use solar energy to make domestic hot water.

(Solar Option Two)

Add one extra domestic hot water heaters for added heat storage. Take advantage of the cooling benefit when you use water.

You will need 5-6 solar collectors. Cost effectiveness will be excellent.

80-90%

Architectural flexibility will be very good. Slab on grade construction will be helpful on the first floor. Solar energy can be stored within the slab, and thermal mass will help with cooling as well. Use a radiant heating system using the domestic hot water heater as the heat source. Then use solar energy to make domestic hot water.

(Solar Option Two)

Add one or two extra domestic hot water heaters for added heat storage.

You will need 4-5 solar collectors and cost effectiveness will be excellent.

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