Solar Water Heating Bob Ramlow

1. Solar Water HeatingBob Ramlow Chapter 7: Choosing The Right System For Your Needs Bruce Hesher Engineering Technology Brevard Community College

2. After doing the conservation and weatherization items that make sense for the situation, it is time to move on to renewable energy. Solar Water Heating is usually done before solar electric because the paybacks are shorter (3-5 vs. 8-10). If solar electric will one day be added, set aside the location for the panels. SWH can tolerate a little shading better than PV systems. Also, once you have enough hot water any more goes unused but, excess electricity can be sold back the utility.

3. Plan for the worst case scenario. If only 2 people are living in a 3 bedroom home, you may still want to size the system for more than 2 people. Before selecting a system, check for stipulations on any rebates you want to apply for. Does the system need to be closed loop anti-freeze, or does it need to be SRCC or FSEC certified? Ask the rebating agency. See www.solar-rating.org/default.htm Physical limitations may also apply. For example, thermo-siphon systems must have the collector below the tank; this is not always possible.

4. Do the shading analysis. Especially important for the winter months when irradiance is low. Make sure the collector is unshaded all year during the 3 hours before and after local solar noon. Azimuth matters but not as much as for PV modules. If you need to locate a collector up to 45° off from due south it will have less than 10% impact at a latitude of less than 30°. Siting a Solar Energy System

5. CAD for Shading Analysis Google Sketchup: Commercial program ($500) with freeware version. Draw 3D model of structure. Geo-locate. Turn on shading. Use month & hour sliders to look for shading.

6. Siting Tools The Solar Path Finder and the Solmetric Sun Eye are valuable tools to make sure a site will not get shaded at different times of the year. For any given location, a solar window can be envisioned that starts low to the horizon on the winter solstice and rises in the sky reaching a peak at the summer solstice and extends from east to west for 3 hours before until 3 hours after solar noon. See p127.

7. Sizing a Solar Water Heating System Over the years some general rules for system sizing have developed. 1) Don’t try to size to meet 100% of the hot water needs. This would raise the cost and make the system big enough for the worst case month, and have excess hot water in others. 2) Start with load and work backwards. The load is the need you are trying to meet.

8. Calculating the Load (SWH) It is possible but expensive and difficult to meter the hot water consumption. Estimation usually is close enough. Figure 20 gal/day/person. However, homes with more people tend to use less per person due to economies of scale (full loads of laundry or dishes). If the home has an over size bath tub or a spa be sure to account for it. If few people live in a home that would normally have more, size the system for the home not the current number of residents. This will enable it to improve the resale value.

9. Sizing the Solar Storage Tank The number of people in the home, the desired days of autonomy, and the average consumption can be multiplied to get the tank size. For every gallon of daily usage you will want a gallon of storage. 3 people x20 gal/day x2 days =120 gallon tank. Since the largest practical solar tank is 120 gallon most installations are done with 80 or 120 gallon tanks.

10. Sizing the Collector Array A system can have more than one collector and there are different sizes of collectors. Knowing the size of the tank and the zone where the system is will allow you to size the collector. In zone 1 (Florida) you need 1.5 to 2 gallons of storage for every square foot of collector space. So, a 120 gallon tank would need a collector with 60 to 80 square feet of collector. A 4’ x 10’ collector works well with a 120 gallon tank in south Florida. Q: Is the collector on the BCC solar trainer sized correctly?

11. Sizing for Evacuated Tube or Concentrating Collectors Most evacuated tube collectors come with a sizing manual or you can calculate the collector size by knowing its efficiency in terms of Btu / ft2/ day for your region (zone). You can find this online through the SRCC. 600-700 Btu/ ft2/day is typical. Convert your hot water load to Btu’s. A Btu is the amount of energy required to raise one pound of water one degree F. A gallon of water is 8.34 pounds. 8.34 Btu’s increases one gallon of water one degree F. Ground water in central Florida is 72°F and hot water needs to be at least 125°F or heated by 53°F. 120g x 53°F x 8.34Btu/g = 53,000Btu 53,000Btu / 650Btu/ft2/day = 81.6ft2/day This is a 10’ by 8’ collector or two 10’ by 4’ collectors. If you can take 2 days to get the water up to temp only one collector would be needed.

12. Additional Sizing Considerations Size the SWH system to the home not how many people currently live in it. SWH systems can last 20-40years. What if the family grows? Babies are messy; they require more gallons of hot water daily than adults. If there will be shading of the array or if the azimuth of the array is large take this into consideration.

13. Sizing a Solar Space Heating System Solar space heating is not popular in Florida. However, it should be. A central heat and A/C unit with strip heating uses a lot of electricity. It is difficult and expensive to do 100% of the heating of a dwelling with solar. Heating takes a lot of energy and there is little solar to work with in winter. Insulation and weatherization is where to put your money. Solar space heating systems without storage can provide up to 25% of the heat for a building, systems with storage can provide up to 50%.

14. Calculating the Load (SSH) Usually done by examining the bill for whatever type of heating is currently used. Natural gas: 100,00 Btu/therm Coal: 12,000 Btu/lb Propane (LP): 91,000Btu/gal Fuel oil: 139,00Btu/gal Electricity: 3,412Btu/KWH Gasoline: 125,000Btu/gal. It is not practical to size a system to meet the heating needs of the coldest winter day; instead target the average winter day.

15. Sizing the Collector Array (SSH) The SSH collector array is in addition to the SWH collector. It can be a 2nd collector or just a larger SWH collector. In northern climates flat plate collectors produce 700-800 Btu/ft2/day. A 32ft2 collector will produce 24,000Btu/day on a clear winter day. Divide the Btu’s needed by the daily Btu’s produced per collector to get the number of collectors needed. Sample Sizing Calculation Natural gas consumption for heating season: 636 therms Days in heating season: 212 Average daily consumption: 6363therms / 212days = 3 therms/day Convert to Btu: 3 x 100,000 = 300,000Btu/day Btu per ft2 of collector: 750 Btu/ft2 Divide total load by collector performance: 300,000 / 750 = 400ft2 of collector. It would take ten 4’ by 10’ collectors to meet 100% of the heating load!

16. Sizing Systems with Storage Complex topic with a lot of variables. The biggest variable is “How much of the heating load do you want to meet by solar?” Another big variable is the heat loss of the home. The 400ft2 collector that heats the house for a day needs a storage tank with 1.25g / ft2 of collector. This is a 500 gallon tank! Easiest approach is to use “rules of thumb”.

17. Rules of Thumb for Sizing Solar Heating Systems Rules of thumb have been developed over years of experience. They are middle of the road estimates. The hardest variable is the heat loss of the building. The rules are for a moderate climate such as Wisconsin where there are 8,500 heating degree days per year. To size dump systems with no storage use one ft2 of collector for every 10ft2 of floor space on the main floor; add 10% for a two story house. This will do 20-25% of the heating. For systems with storage use 2 ft2 for every 10ft2 of main floor to meet 50% of the heating load. You can’t make a system big enough to compensate for a December with only 5 sunny days no matter how much money you throw at it.

18. Sizing with Computer Analysis Software Software that does the calculations and asks the right questions is convenient, fast, and accurate but, it is also expensive and not widely distributed. they also enable doing ”what if” estimates by changing variables. If you are going into the solar business get some software. If you are looking at doing your own home use rules of thumb.

19. Sizing Combination Systems A combination system that produce hot water and heats the home can take advantage of some economies of scale. If you calculated 8.5 collectors for space heating and 2 for water heating you would need 10.5 collectors. The reduction in losses due to economies of scale in piping and etc. can reduce the collectors by 25%. So, the same home would need 8 collectors.

20. Sizing ComponentsSizing Pumps There are 2 important parameters for pumps: Flow (how many gallons per minute and Head (how much water can the pump move vertically up). If the collector is above the tank, the pump must be strong enough to overcome the head of the volume of water from the tank to the collector. Once primed the pump only needs to circulate at which time the flow is the parameter of interest.

21. Pipe Sizing Pipe sizing is determined by the flow. Pipes that are too small will reduce the flow rate. Pipes that are too large cost more and can be harder to plumb. Large pipes also hold more heat transfers fluid which adds to the cost. The rule of thumb is to use ¾” pipe for 4 or fewer collectors and 1” for up to 8 collectors. ¾” pipes have atypical flow rate of 4gpm and 1” pipe have a flow rate of 8gpm. Too large a pipe is better than too small a pipe. It reduces back pressure on the pump but it also requires more head due to the increased volume of water in the pipe going up to the collector.

22. Fluid Volume Estimation The amount of solar fluid in a system will be a factor of the size of the piping, the length of piping and the fluid requirements of the collector(s), heat exchanger, and any other components. Flat plate collectors hold from 1.25 to 2 gallons each. See page 156 for gallons of fluid per foot of pipe. Read the docs/datasheet of all component used and total up the fluid requirements.

23. Sizing Expansion Tanks An appropriate size expansion tank is need on all pressurized systems. A typical Florida direct/active system like the BCC trainer is pressurized by the utility water pressure. The tank size is determined by the volume of fluid in the system. If the tank is too small it will not account for all of the thermal expansion in the system. If the pressure drops too low air may be sucked into the system. See page 157 for expansion tank sizes

24. Sizing Air Heating Systems Do not oversize the collector. A balance between collector heat output, building heat loss, and thermal mass of the building. In cold climate the rule of thumb is the collector area should be 10% of the main floor space. During winter the collector will only work for about 4 hours a day.