To determine the proper model and quantity of collectors for a solar thermal system the water heating load must first be determined. Once the heating load is determined the solar collector array can be sized to meet the entire or a fraction of it. For residential systems on single-family homes the figure below can be used to determine system size.

Region | # of People | System | Collector Area | System Capacity |
---|---|---|---|---|

A & B | 4 | 40 / 80 | 40 sq. ft. | 80 gal. |

A & B | 4 – 5 | 64 / 80 | 64 sq. ft. | 80 gal. |

A & B | 6 | 64 / 120 | 64 sq. ft. | 120 gal. |

A & B | 7 | 80 / 120 | 80 sq. ft. | 120 gal. |

C, D, & E | 4 | 40 / 80 | 40 sq. ft. | 80 gal. |

C, D, & E | 4 – 5 | 64 / 80 | 64 sq. ft. | 80 gal. |

C, D, & E | 6 | 64 / 120 | 64 sq. ft. | 120 gal. |

C, D, & E | 7 | 80 / 120 | 80 sq. ft. | 120 gal. |

F | 4 – 7 | 80 / 120 | 80 sq. ft. | 120 gal. |

For solar thermal systems used in a domestic hot water application the hot water usage in gallons/day (GPD) needs to be determined. This can be done using site-specific data or using maximums and/or averages published by ASHRAE for various type of facilities.

Facility Type | Average Daily Draw | Maximum Daily Draw |
---|---|---|

Hotels / Motels | ||

20 units or less | 20 GPD / unit | 35 GPD / unit |

60 units | 14 GPD / unit | 25 GPD / unit |

100 units or more | 10 GPD / unit | 15 GPD / unit |

Nursing Homes | ||

18 GPD / bed | 30 GPD / bed | |

Dormitories | ||

Men | 13 GPD / student | 22 GPD / student |

Women | 12 GPD / student | 26 GPD / student |

Apartment Buildings | ||

20 units or less | 42 GPD / unit | 80 GPD / unit |

50 units | 40 GPD / unit | 73 GPD / unit |

75 units | 38 GPD / unit | 66 GPD / unit |

100 units | 37 GPD / unit | 60 GPD / unit |

200 units or more | 35 GPD / unit | 52 GPD / unit |

Office Buildings | ||

1 GPD / person | 2 GPD / person | |

Restaurants | ||

Full Service | 3 GPD / meal | 11 GPD / meal |

Fast food, Sandwich shops, etc | 1 GPD / meal | 6 GPD / meal |

Schools | ||

Elementary | 1 GPD / student | 2 GPD / student |

Middle & High | 2 GPD / student | 4 GPD / student |

### Converting Domestic Hot Water Usage to Water Heating Load

Once the daily hot water usage has been determined the temperature difference must then be calculated. This is done by taking the desired hot water setpoint temperature (usually 130-140 F) and subtracting the mains water temperature from that. The mains water temperature is the temperature at which water enters the building from the utility and can be determined by using the average groundwater temperature for the area in question. To heat one gallon of water up by 1°F requires 8.33 BTUs of energy so the total heating load can be found by multiplying the gallons/day usage found by the temperature difference and then multiplying the product of that by 8.33. For example, if we needed to determine the daily water heating load for 20 unit apartment building in Orlando, FL using the average daily hot water usage we would calculate the following:

8.33 BTU/gal-F * [(42 GPD/unit * 20 units) * (140°F – 72°F)] = 475,809.6 BTU/day.

### Determining Collector Area Required based on Water Heating Demands

To safely and optimally determine the size of solar thermal collector array the calculated water heating load should be matched to the expected output of the solar array. For initial sizing purposes we can use the rating published by SRCC for Category C, High-Radiation conditions. The Solene AR Series is rated at 1095 BTU/ft^{2}-day and the Solene SG Series is rated at 1070 BTU/ft^{2}-day. In reality the power output of the solar thermal collectors varies constantly with solar insolation and weather but using the figures as noted helps ensure that the system is sized so that it will not stagnate during the warmest, brightest months and possibly cause issues due to extreme temperatures and pressures. A multiplier of 1.20 should factored into the sizing result to make up for real-world energy losses from piping, tanks, etc. Using the previous example of the apartment building, and assuming the Solene SG Series collector will be used, the system would be sized as follows: 1.20*[(475,809.6 BTU/day) / (1070 BTU/ft^{2}-day)] = 533.6 ft^{2}. The calculation yields a result of 533.6 ft^{2} which would equate to (14) SLSG-40 collectors. It should be noted that the collectors will not perform at this level 100% of the time but, conservatively, should yield a solar fractions of 50-75% over the course of a year.