Information Jo Sheet 4 System Sizing Solar heating system sizing To determine the size requirements for each system component in a solar heating system, it is necessary to estimate the system load. The amount of hot water needed for a particular household is ased upon how much hot water is typically used per day. An average modern household uses aout 75 L of hot water per person per day. Of course, this estimate can vary consideraly for different families and various circumstances, such as climate for the area. Once the average daily hot water load is known, you can egin sizing each system component. When determining your daily hot water load, consider the numer of edrooms in the house and future possiilities, such as new family memers. Solar storage tank size (volume) For domestic hot water (DHW) storage, the tank should hold the average daily consumption (in L) for that residency. Therefore, using the previously mentioned average volume of 75 L per person per day, a four-person home might require a 300 L storage tank. If necessary, you can round up the calculated storage capacity to the nearest standard tank size. For example, a single-person homeowner could install a 115 L or 150 L tank. Solar collector array size (area) For a solar water heating system (SWHS), the solar irradiance for a given area directly impacts the ratio of collector area to storage capacity required to heat all the water in the tank. The Earth is graded in different solar zones depending on the average solar irradiance per day, as shown in Figure 39. This map can e used to find the daily sum of solar irradiance for a given zone. Tale 3 gives the ratio etween tank size and array size for different solar zones. Festo Didactic 52667-20 51
Long-term average of daily sum < 2.0 3.0 4.0 5.0 6.0 7.0 >7.5 kwh/m 2 Figure 39. World irradiance map per day (SolarGIS 2016 GeoModel Solar). Tale 3. Tank/Array size ratios (in L/m 2 ). Daily sum irradiation (kwh/m 2 day) Tank size to array size ratio 0 to 2 0 to 20 2 to 3 20 to 31 3 to 4 31 to 41 4 to 5 41 to 51 5 to 6 51 to 61 6 to 7 61 to 71 7 to 8 71 to 81 Typical solar irradiance in a sunny climate provides aout 6 kwh/m 2 day, and a cloudy climate provides aout 3 kwh/m 2 day. Therefore, in a sunny climate, a 100 m 2 array produces aout 315 kwh/day. Sizing example For a four-person home in the Mid-Atlantic region (aout 4 kwh/m 2 day), the 300 L storage capacity is divided y 41, resulting in a solar array size of aout 7.3 m 2. Common solar panels measure 1.2 m x 2.4 m (2.88 m 2 ) and 1.2 m x 3 m (3.6 m 2 ). Therefore, two parallel-connected solar panels are required in the array. 52 Festo Didactic 52667-20
In the case of the solar thermal training system, the solar panels measure 1 m x 1.15 m. Therefore, six of these solar panels would e required (1.15 m 2 x 6 = 6.9 m 2 ). However, the actual asorer area of the solar panel is only 1 m 2 (x 6 = 6 m 2 ). This total area (6 m 2 ) is equivalent to approximately 37 m 2 of photovoltaic solar modules, or 4 kw peak power of electricity. Another common guideline for residential SWHS installations recommends using 1.9 m 2 of solar collector area for each person (up to two persons), plus 0.8 m 2 per additional person in sunny climates or 1.3 m 2 per additional person in cloudy climates. Using this guideline, the required solar panel area for a household of four persons is equal to 5.4 m 2, which is comparale to the 6 m 2 area otained previously. If the solar collector array is partially ostructed from the sun or is facing more than 30 degrees away from due south, the solar collector area should e increased accordingly. Thermal capacity Many solar collectors are shipped with sizing instructions for a given load and geographical location. However, you can also use the solar collector thermal capacity (if specified y the manufacturer) to determine the proper array size. The solar collector on the training system has a thermal capacity rating of 30.6 kj/m 2 (0.85 Wh/m 2 ), which equals a thermal energy output of 2.36 kwh/m 2 day. You may also find the thermal energy output (kwh/m 2 day) that the collector will produce for the location y visiting the Solar Rating & Certification Corporation (SRCC) wesite (www.solar-rating.org). Calculate the thermal energy output (in kwh/day) for the entire solar collector array y multiplying the total array area (in m 2 ) y the thermal energy output value in kwh/m 2 day. Tale 4. Typical solar collector thermal energy output. Collector type Flat plate Evacuated tue Thermal energy 2.21-2.52 kwh/m 2 day 1.89-2.21 kwh/m 2 day For solar space heating systems (SSHS), climate is the iggest factor on system sizing. In most climates, clouds can cause extended periods without sufficient sunlight, which prevents heating a dwelling with 100% solar energy. Building heat loss is the second largest influence on system sizing. Systems without thermal storage can only provide aout 25% of the annual space heating load, while systems with thermal storage can provide up to 50% on average. Systems with high-mass thermal storage can provide 75% or more, even in cold climates. All uildings lose heat to cooler outdoor temperatures at some rate that can e measured. Well insulated homes lose heat slowly, compared to non-insulated uildings, such as a tool shed or garage. Energy efficient construction practices help to reduce heat loss and conserve energy. Long term, this effort is usually less costly than uying more energy. Festo Didactic 52667-20 53
Doors 11% Windows 10% Fans, vents 4% Electrical outlets, switches 2% Floors, walls, ceiling 30% Pluming 13% Fireplace 14% Air ducts 15% Figure 40. Building air leakage. A uilding s heating load is more complicated to estimate than a hot water load, ecause there are many additional variales. For example, the heating season varies differently for each geographical location. The numer of days in the heating season for your area can often e found y checking your conventional heating ill. This value can e used to determine the space heating load of your house, ased upon heating degree days. The heating degree day (HDD) is a unit of measure for determining thermal energy requirements for space heating. For any given day, the heating degree day specifies how many degrees the outdoor temperature was elow an indoor ase temperature of 18.33 C for a specific location. Basically, the amount of energy needed to heat a uilding throughout a given time period (heat load) is directly proportional to the heating degree days in that period. You can also find heating degree days (HDDs) or cooling degree days (CDDs) for your area, y visiting: www.degreedays.net. a The cooling degree day (CDD) is used to measure thermal energy requirements for air-conditioning. Cooling load (approximation) (in kw/h) = airspace volume (in m 3 ) x 3 If a heat load analysis is performed on your home y a professional, the resulting value is often specified in kw/heating degree day m 2. This value should e multiplied y the total floor area inside your house (in m 2 ) to determine kw per heating degree day (kw/hdd). You can check with your local weather service or utility company to find the numer of heating degree days in one year, and divide y the numer of days in your heating season to determine the average heating degree days per day. Multiply this value y the kw/hdd value to determine your heat load in kw/day. Tale 5 compares the thermal output (kw) of various heating fuels. 54 Festo Didactic 52667-20
Tale 5. Fuel comparison. Fuel type Coal Electricity Fuel Oil Gasoline Natural gas Propane Thermal value 7.75 kwh/kg or 27 912 kj/kg 3600 kj/kwh 10.76 kwh/l or 38 741.56 kj/l 9.68 kwh/l or 34 839.54 kj/l 277.84 kwh/gj 7.05 kwh/l or 25 363.18 kj/l Wood 1374.58 kwh/m 3 or 4 948 472.37 kj/m 3 Heat load example The following heat load example is ased on only one month from a single monthly utility ill; however, averaging several months or even years provides more valuale data. 2930.71 kwh / 30 days or 10.55 GJ /30 days= 97.69 kwh/day or 0.35 GJ/day The next heat load calculation shown is ased on one entire year of monthly utility ills. This example is more accurate due to monthly averaging, and it also uses heating degree days to determine the heat load. 29 307.11 kwh / 4000 HDD or 105.51 GJ/ 4000 HDD = 7.33 kwh/hdd or 26.38 J/HDD 4000 HDD / 160 days = 25 HDD/day 25 HDD/day x 7.33 kwh/hdd = 183.25 kwh/day Once you have determined your home space heating load (in kj per day), you can check the solar collector manufacturer specifications to size the final array. For example, a 5.95 m 2 flat-plate array will collect aout 14.07 kwh/day. Estimating system size Undersizing is actually etter than oversizing the solar collector array, ecause every it of solar power helps, ut too much can quickly overheat the system and create uncomfortale living conditions. A larger system also costs more money. Thermal storage complicates the system sizing task, as well. Detailed design considerations include array size, storage size, expansion tank size, pipe size, and heat exchanger size. A simple rule-of-thum is to size the storage tank for 4.73 liters (etween 3.79 and 7.57 L, as shown in Tale 3) per square meter (m 2 ) of solar collector area. This value assumes 24 hours of heating for a day of Festo Didactic 52667-20 55
average solar conditions. For solar heating systems with no thermal storage, you can use 0.09 m 2 of collector area for every 0.93 m 2 of main floor area in the house and add 10% to the array size for a two-story uilding. This simple sizing method should provide aout 20-25% of annual solar heating contriution. With thermal storage included, use no more than 0.19 m 2 of collector area for every ten 0.93 m 2 of floor area. You can expect up to 50% of solar contriution to your annual heating load. This value is well elow 100%, mostly due to cloudy days during the heating season. When comining solar water and space heating, add the two solar collector array areas required, ut reduce the array size slightly due to shared resources that may reduce overall thermal losses. Other sizing resources You can also size a solar thermal energy system and check home energy efficiency y using freely availale computer software. Some free programs are listed elow. EnergyPlus from DOE/EERE (http://www.energyplus.gov/) HEED from UCLA (http://www.aud.ucla.edu/heed/) SUNREL from NREL (http://www.nrel.gov/uildings/sunrel/) Rememer that the sun is a variale resource, so long-term averaging of historical data must e used to predict overall system performance. Our weather patterns continually change from one year to the next. By using computer software simulation, you can validate your preliminary system size estimation and potentially improve the accuracy of your component sizing. 56 Festo Didactic 52667-20
Jo Sheet 4 System Sizing OBJECTIVE In this jo, you will learn how to estimate the proper size of the system components for an efficient solar thermal energy system. PROCEDURE 1. A small, three-person, single-story home located in New Jersey (North East of the United States) has the dimensions shown in Figure 41. 12 m 3 m 6 m Figure 41. House footprint. 6 m 2. What is the floor area of the residential uilding? Floor area: m 2 a With your instructor s approval, you may answer all questions in this Jo Sheet with data from your own residence. Otherwise, continue to use the information provided in the example. 3. Referring to Figure 42, record the average daily sum irradiation for the geographical location of the house. kwh/m 2 day Festo Didactic 52667-20 57
Long-term average of daily sum < 2.0 3.0 4.0 5.0 6.0 7.0 >7.5 kwh/m 2 Figure 42. World s solar irradiation map. 4. The 50-year old house contains a comination heating system that uses a natural gas furnace for space heating and a natural gas 189.27 L water heater to supply domestic hot water (DHW). The home also uses a natural gas clothes dryer, ut this function will e replaced y a solar dryer and/or a clothes line. 5. Based on the tank/array size ratio from Tale 6 for the average daily sum irradiation previously determined, what is the area of the solar collector array that would e required just to heat the water? Array area: m 2 a Additional solar collector area is required to provide space heating as well. 58 Festo Didactic 52667-20
Tale 6. Tank/array size ratios (in L/m 2 ). Daily sum irradiation (kwh/m 2 day) Size Ratio 0 to 2 0 to 20 2 to 3 20 to 31 3 to 4 31 to 41 4 to 5 41 to 51 5 to 6 51 to 61 6 to 7 61 to 71 7 to 8 71 to 81 6. The monthly natural gas ills for the entire year were recorded in Tale 7. Tale 7. Monthly gas ill values. Jan Fe Mar Apr May Jun Jul Aug Sep Oct Nov Dec kwh 6092 5965 5108 3287 830 1353 246 1355 215 984 1108 4639 Billing days Degree days 35 33 31 29 31 29 28 29 30 31 27 34 1074 1185 707 471 93 2 0 0 6 28 271 769 7. If the heating season for the location is from Novemer to May, how many kwh were used during this period? kwh 8. Convert kwh to kj, and record the value. 1 kwh = 3600 kj) kj 9. How many days were illed during the heating season period? Days Festo Didactic 52667-20 59
10. Based on illing days, what was the home s heat load for the heating season? Divide Btu y numer of days illed. kwh/day or kj/day 11. How many heating degree days are in this heating season period? Heating degree days 12. Divide annual kwh (or kj) for the heating season y the heating degree days in the heating season to determine Btu per heating degree day. kwh/hdd or kj/hdd 13. Calculate the numer of heating degree days per day for the heating season. Divide heating degree days y numer of days in the heating season. Heating degree days/day 14. Based on degree days, what was the home s heat load for the heating season? Multiply kwh/hdd y heating degree days/day. kwh/day or kj/day 15. By using the second (degree days) method to calculate the home s heat load, were your results similar to the first (illing days) method that you used to calculate the home s heat load? a Yes No The solar collector array will e used for heating oth the water and space in this comination solar heating system. 16. Based on the floor area of the uilding and y using some general rules-ofthum (provided earlier), what is an estimated size for the solar collector array if a non-storage space heating system is eing planned? If you determined a range of possile sizes, take the average of your estimated array size range. m 2 60 Festo Didactic 52667-20
17. Add the array area required to heat water to your estimated array size for space heating, and record your answer. m 2 18. How many 1.22 m x 3.05 m (3.72 m 2 ) solar collector panels would e required for this solar heating project? Divide your estimated array size y the area of one collector panel and round up to the nearest whole numer. Panels 19. What is the total array area using the previously determined quantity of solar collector panels? m 2 20. Considering that flat-plate solar collectors produce a thermal energy output of aout 2.37 kwh/m 2 day, what is the daily kwh output (in kwh/day) for your array? Multiply the total array area y 2.37 kwh/m2 day. kwh/day 21. The average solar irradiance in the state of New Jersey (40 N latitude) is aout 3.93 kwh/m 2 day. Based on this value and assuming typical flat-plate solar collector efficiency of more than 60% (with up to 10 C rise from amient), will the solar collector array have enough radiant solar energy availale to heat the solar collectors to their full thermal output rating of 2.37 kwh/m 2 day? Yes No 22. What is the total array area required for space heating only? Remove one solar collector panel used for heating water. m 2 23. You can verify that this solar collector array size is adequate y comparing the expected solar heating contriution to the heat load of the conventional heating system in the home. Festo Didactic 52667-20 61
24. What is the daily kwh output for the space heating portion of your array? Multiply the total array area required for space heating y 2.37 kwh/m2 day. kwh/day 25. Compare the space heating array output (in kwh/day) to your calculated heat load (in kj/day). What is the percentage of solar contriution to the home s space heating system? % 26. Assuming that no thermal storage is used for space heating, is this solar contriution etween the expected values of 20% and 25%? Yes No 27. What is the maximum solar collector array area that would e required for space heating with thermal storage, excluding hot water demands? m 2 28. What size storage tank would e needed for this array size? L Name: Date: Instructor's approval: 62 Festo Didactic 52667-20