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Understanding Thermal Energy in Solar Hot Water Systems
SolarWave measures and reports energy in five different ways:




1. Load: Hot water demanded


This is the amount of thermal energy used

by the hot water systems of the building. We

measure the percent of the load met by solar


2. Solar Resource: Available solar thermal


This is the maximum amount of thermal

energy available to the collectors, given the intensity of sunlight and the ambient

temperature. This is used to evaluate the efficiency of the system and optimize its production.

3. Solar Production: Thermal energy generated by collectors

When the sun hits solar collectors, they generate heat. When the collectors are hotter than a storage tank a pump moves fluid from collector to a storage tank, where heat is stored until needed. Solar Production is measured with a flow-meter and 2 temperature sensors (supply & return) providing an effective BTU-meter measuring energy collected or produced. An indicated production measurement can be done using a collector sensor instead of a return sensor, or an estimated flowrate. These indicated production measurements are not as accurate. 

4. Solar Loop Supply and Return Temperatures (Cold and Hot)

The Solar Loop Supply is the supply of water from the tank to the collectors, and is referred to as the "Solar Loop Cold" Sensor. Cold is used because while the system is operating this temperature should be colder than the return temperature. 

The Solar Loop Return is the return from the collectors to the tank, and is referred to as the "Solar Loop Hot" Sensor. Hot is used because while the system is operating this temperature should be hotter than the supply temperature. 

The terms Hot and Cold are used to avoid confusion as supply and return are used interchangeably in various types of heating and cooling systems. 


5. Solar Contribution: Thermal energy delivered to Hot Water or heating of a building

When a hot water tap is turned on, hot water flows from the storage tank to meet the load. If no energy is used or required there is no contribution. The water loses heat while it is being stored, so solar contribution is generally lower than solar production. If there is little or no load in a given period, it can be significantly lower. In a solar hot water system the “contributed” solar energy or BTUs are measured with a flow-meter and 2 sensors (cold inlet and hot outlet of a solar storage tank, possibly a preheat tank).

6. Solar Fraction:  Solar fraction is the percentage of solar contribution to the total energy load (measured in BTUs).

7. Displaced Energy: Fossil fuel that would be required to replace solar contribution

Solar hot water systems are paired with a conventional gas, oil or electrical system which supplements delivery of hot water when solar cannot meet the load. Fossil Fuel Displacement measures the fossil fuel that would be required by the conventional hot-water system to meet the load that is instead being met by solar energy. Ultimately, the value of the solar contribution is the fossil fuel displaced. Displaced energy is calculated by dividing  the Solar Contribution by the efficiency of the conventional heating system. Additionally the volume of fuel displaced can be calculated from this value as well.  

8. Solar Constant: The amount of energy from the sun that strikes the earth’s atmosphere. 1366 W/sq meter (429 BTUs/ sq ft) (in space) ; 1100 W/sq meter (310 BTUs/sq ft) clear sky( desert conditions). The intensity of sunlight and ambient temperature is used to evaluate the efficiency of the system and optimize production.

9. Seasonal Efficiency: The total efficiency for a heat applicant, such as a boiler or hot water heater, where it is a ratio of the percent of water BTUs delivered to the load (tap) to the amount of fuel BTUs consumed. Example: if a hot-water heater had a seasonal efficiency of 75% it would require 100 units of fuel energy to produce 75 units of water energy. So, delivering 75,000 BTUs from a solar tank to a back-up heater with a seasonal efficiency of 75% would displace 1 Therm (100,000 BTUs) thereby producing a value of the displaced rather than just the produced quantity.



British Thermal Unit (BTU) energy required to raise 1 lb. water (+-pint) 1 degree F.

1 Therm = 100,000 BTUs
1 gallon #2 fuel oil = 140,000 BTUs
1 gallon propane = 91.300 BTUs
1 kWh= 3413 BTUs
1 Therm= 29.3 kWhs (29.29974)
1 sq ft south window (Boston) = 125,000 BTUs/yr
1 gallon (water)= 8.34 pounds

Heating 100 gallons of water every day from 50° F to 110° F requires 50,040 BTUs/day and 18,264,600 BTUs/year
[100 x 8.34 x (110-50)=50,040 BTUs X 365= 18,264,600 ]

“Load” or “energy load” in terms of energy load for a building’s heating, cooling or hot water demand is measured in units of energy such as BTUs, Therms, kWhs etc.

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