Solar Thermal Energy Ð Delivering the Heat Tom Lane copyright 1992 Tom Lane Catching the sun with solar hot water collectors is a good investment for most homes in the United States. Solar water heating is a great investment compared to electric or Liquid Propane (LP) gas water heating if the rate exceeds $0.07 a Kilowatt-hour or $1.15 a gallon for LP gas. It is not cost-effective versus natural gas. As part of a new home mortgage or home improvement loan, the after-tax rate of return on investment will typically be over 18% per year, even for two people. The monthly savings will be far greater than the small increase on the home mortgage or home improvement loan. Besides increasing the equity in your home, the best part is that a dollar saved on hot water is typically equivalent to $1.30 taxable income. The interest on monies borrowed from the bank is a tax write-off as an itemized deduction. It is like buying real equity instead of renting a service from the utility. Solar hot water is now cost effective in grid-connected homes, unlike solar electric power. However, a buyer needs to understand the rating of solar thermal collectors and the efficiency of solar thermal collectors. System efficiency depends on adequate storage capacity and time of day you use the hot water. Converting BTU's to Watts All solar thermal collectors and systems are rated in British Thermal Units (BTU's). One BTU is the amount of energy it takes to raise one pound of water one degree Fahrenheit. There are 8.34 pounds in a gallon of water, so it takes 8.34 BTU's to raise one gallon one degree fahrenheit. To convert BTU's to watts-hours, the common electrical energy term, multiply the BTU rating of a solar system or solar collector by 0.2931. This multiplier will convert BTU ratings of solar collectors or solar systems directly into watt-hours for a better comparison in energy consumption and production. With this figure you can calculate how many kilowatt-hours (Kwh) a solar system will produce over the period of one year (divide by 1000 and multiply by 365). Then multiply the Kwh's generated per year by your local utility rates, which range from $0.07 - $0.15 per kilowatt hour. This gives a rough idea of how much your system will save in utility bills over the period of one year. That savings will directly relate to your investment in solar hot water and make you an intelligent shopper for good value. SRCC and FSEC Ratings There are two agencies that rate solar thermal collectors. One, a branch of the National Solar Energy Industries Association, is known as the Solar Rating and Certification Corp. (SRCC), 777 N. Capitol St. Ste. 805, Washington, DC 20002, 703-524-6045. Send $35 for their ratings of all thermal collectors and systems they have tested. The other is the Florida Solar Energy Center (FSEC), 300 State Road 401, Cape Canaveral, FL 32920, 407-783-0300. Their Thermal Performance Ratings Catalog is free. They will also send you, upon request, a one page summary information sheet on a specific system or individual collector. FSEC is presently the only lab nationwide that is testing solar thermal collectors. Both SRCC and FSEC ratings are based on all day, clear sky conditions similar to the rating condition for solar electric modules. System Ratings Sometimes you can get a total system's rating from a particular manufacturer, especially on passive hot water systems. For example, the Copper Cricket is rated at 22,000 BTU/day and the PT-40-CN by Thermal Conversion Technology is rated at 24,208 BTU/day by the SRCC. Let's convert these ratings to kilowatt/hrs by multiplying by 0.2931 and dividing by 1000. The Copper Cricket uses sun energy the equivalent of 6.44 Kwh per day, and saves 6.44 Kwh/day from the electric bill. Likewise, the PT-40 saves 7.09 Kwh/day. How does the cost of a system compare to the savings per year? Assume 365 clear sky days per year and a cost of $0.10 a Kwh from the utility. Copper Cricket = 6.44 Kwh/day x 365 x .10 = $235 per year. PT-40 = 7.09 Kwh/day x 365 x .10 = $258 per year. Let's look at system cost (not including installation cost) divided by savings per year to get simple payback. In this example, we won't consider the money value of time, the increasing fuel cost, or that money saved is worth more than money earned. The Copper Cricket sells for $2180 and needs a 52 gallon tank which sells for $160 for a total of $2340. The PT-40 sells for $1500 and if we add a 52 gallon tank for $160 it would cost $1770. Copper Cricket = $2340 divided by $235 for a 9.95 years simple payback. PT-40 = $1760 divided by $258 for a 6.82 years simple payback. These figures can be modified by adding the cost of installation to the system cost before you divide by savings. You can also reduce the savings because there are not 365 clear days in a year. However, figuring simple payback enables you to rate system cost vs. expected savings so comparison among the various solar water heaters can be made. Collector Ratings The Thermal Performance ratings by FSEC is a good approximation of what a thermal collector can deliver daily in savings if system information is unavailable. (see figure below) For example, an American Energy Technologies, Inc. (AET) collector tested at the intermediate temperature rating is rated at 38,100 BTU/day. The 38,100 BTU/day x 0.2931 = 11,167 watts-hours/day which equals 11.17 Kwh per day. If you pay $0.10 per Kwh for electricity, this means a savings of $1.12 a day or $408.80 per year ($1.12 x 365). Two 4 x 10 foot (2 x 40 square feet) collectors would double the savings provided that the following conditions are met. First, the storage tank installed should be adequate. Second, the system was sized for five to seven people who would typically need 80 sq. ft. instead of only two or three people who would typically need only 50 sq. ft. LIKE INSERT THE THERMAL PERFORMANCE RATINGS HERE Solar collectors and solar systems are tested under ideal clear sky conditions to maximize BTU production. For active open-loop systems and active closed-loop systems that use a heat exchanger, the best way to get the most BTU's for your money is to have adequate storage. You need 1.5 to 2.25 gallons of water for each square foot of collector area to maximize collector efficiency. Solar storage tanks are manufactured in 80, 100, and 120 gallon sizes, and solar collectors are manufactured in 4 x 6.5 (26 sq. ft.), 4 x 8 (32 sq. ft.), and 4 x 10 (40 sq. ft.) sizes. Matching collector area to tank size and finding the ratio that falls within a system's optimum range is relatively straight- forward. For example, take two tanks, one 80 gallon and one 120 gallon, both filled with 70¡F water. A single 4 x 8 (32 sq. ft.) collector under testing will raise the 80 gallon tank 55 degrees up to a temperature of 125¡F; the ratio of 2.5 gal per sq ft is high Ð not enough collectors for the amount of storage. Two 4 x 8 (64 sq ft) collectors will raise the 80 gallon tank 85 degrees up to a temperature of 155¡F; the 1.25 ratio is low Ð not adequate storage for amount of collector area). If a 120 gallon tank had been used with the two 4 x 8 collectors (ratio of 1.88), it would raise the 120 gallons a total of 80 degrees up to a temperature of 150¡F. The ratio is 1.87 Ð just right. Let's look at increasing storage size and see what this means in equipment cost and dollars saved. Equipment Specifications 80 gallons of water at 70¡F with 64 sq. ft. collector area gives 1.25 gals/sq ft ratio and temperature rise of 85¡ F per day. 120 gallons of water at 70¡F with 64 sq. ft. collector area gives 1.87 gals/sq ft ratio and a temperature rise of 80¡ F per day Calculations For 80 gallons of water at 70¡F with 64 sq. ft. collector area (1.25 gals/sq ft ratio) and temperature rise of 85¡ F per day, the savings will be $605.90 per year. 80 gals x 8.34 lbs/gal = 667 lbs, x 85 degrees = 56,712 BTU's, x 0.2931 factor = 16,622 watt-hours, Ö 1000 = 16.62 Kwh, x $0.10 (equivalent to LP gas at $1.60 per gallon) = 1.66 cents/day, x 365 = $605.90 per year. For 120 gallons of water at 70¡F with 64 sq. ft. collector area (1.87 gals/sq ft ratio) and a temperature rise of 80¡ F per day, the savings will be $854.00 per year. 120 gallons x 8.34 lbs/gal = 1001 lbs x 80 degrees = 80,064 BTUs x 0.2931 = 23,466 watt-hours Ö 1000 =23.47 Kwh x $0.10 (equivalent to LP gas at $1.60 per gallon) =$854.00 per year. Comparisons The 120 gallon tank typically costs $120 to $150 more than an 80 gallon tank and will give you a return on that investment in less than one year!! The $854 saved would be an example of simple payback for an active open loop system. An open loop solar water heater is one where the water in the tank is heated directly by the solar thermal panel on the roof without using a heat exchanger. If we use a closed loop solar water heater, less than $854 would be saved due to efficiency losses in the heat exchanger. This reduction would be by 5% if a double pump counterflow external heat exchanger was used, by 15% to 20% if the tank had the heat exchanger built into the tank walls or inside of the tank, and by 35% if an external thermosiphon heat exchanger was used. Another loss to consider is if your system uses an ac pump and controller instead of a DC pump and PV panel. In this case you can subtract an additional 8% from the $854 noted above. If we use the active closed-loop system with an integral heat exchange tank with a DC pump and solar electric panel (described in Home Power #25), then the total system cost would be about $1800. The $854 saved would be reduced to $683 for the integral heat exchanger tank losses compared to an open loop system. If we spent an extra $150, increasing the system cost to $1950, and used two 4 x 10 collectors instead of two 4 x 8 collectors, the additional heat delivered to the heat exchanger would overcome the 20% savings reduction (heat exchanger efficiency loss). RICHARD PLEASE SEE HC FOR FORMULAS Insert Equation 1 IMPORTANT NOTE: Installation costs and realistic projections for weather conditions will typically double the simple payback time. Understanding Thermal Efficiency Curves Simply looking at a SRCC or FSEC collector ratings does not mean that the collector will put those BTU's into storage. The storage may be inadequate or the heat exchanger may be inefficient. There are three factors involved in the efficiency formula for a solar thermal collector. RICHARD SEE HC FOR FORMULA Enter Equation 2 I is the insolation or amount of sunlight hitting the collector (measured as power per area). Ta is the ambient air temperature. Ti is the temperature of the fluid entering the solar collector, called the inlet temperature,. The efficiency drops as the ambient air temperature drops or the entering water temperature goes up or both. You obviously cannot change the ambient or outside air temperature Ta, or the amount of incoming sunlight, I. We can affect Ti or the inlet temperature by several strategies. Increasing the Efficiency of Solar Water Heaters 1) Keep the storage-to-collector ratio within 1.5 to 2.25 gallons per square foot. For space heating the best ratio is 1.75 to 3 gallons per square feet. 2) Do laundry or dish washing between 9 AM and 3 PM using hot water while the sun is still shining on your collector. This will increase BTU's collected each day by lowering the Ti. In fact the ratio of collector area can drop to 1.25 gallons to square foot if at least 50% of your storage is used from 9:00 AM to 3:00 PM. This strategy is usually found in commercial applications where the load occurs during prime solar time. If you cannot find any collector ratings for the collector you are considering buying, under a clear sky the maximum any flat plate or evacuated tube solar collector can generate is about 970 BTU/sq ft/per day. Summary Just as proper battery storage and inverter losses must be figured for solar electric systems, factoring volume storage to thermal collector ratios, and heat exchanger efficiencies achieves the proper system balance. A common mistake is buying a system that is too small. The installation cost will be essentially the same whether it's a 52 gallon tank with 32 sq. ft. collector or a 120 gallon tank with 80 sq. ft. collector. Plan on 20 gallons per person and 14 sq. ft. of collector area per person. As an investment, stick with a minimum of 80 gallons and 40 sq. ft. on an open loop active system for two people; allow 52 sq. ft. on any system that uses a heat exchanger. A solar system offers you real savings and real equity for your money instead of an overpriced service from your local utility to heat your hot water. The choice is not about spending money, but what you are getting in return for a monthly investment you are already making. We will eventually learn about the investment value of solar thermal and learn to think like accountants instead of dazed consumers. Then, we will be on our way to saying "grid riddance" to utility companies and starting to repair the environmental damage to our planet. Access Author: Tom Lane, Energy Conservation Services of North Florida Inc., 4110 SW 34th St. Ste. 15, Gainesville, FL 32608, 904-373-3220