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 energy.
2. Solar Resource: Available solar thermal energy
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
When the sun hits solar collectors, they generate heat. This thermal energy is generally used to heat the water in a storage tank, where it is stored until needed. Solar Production is the thermal energy actually generated by solar hot water collectors and delivered to the tank.
4. Solar Contribution: Thermal energy delivered
When a hot water tap is turned on, hot water flows from the storage tank to meet the load. 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.
5. Fossil Fuel Displacement: 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.
Some terms and units used
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
So, 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.
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.
Solar Production: When the sunlight strikes solar collectors, they heat up. 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. In a solar hot water system the “produced” energy is the energy that is delivered from the panels on the roof into the building. Often “Produced BTUs” would be measured with a flow-meter and 2 sensors (supply & return) providing an effective BTU-meter measuring energy collected or produced.
Solar Contribution refers to the amount of useful energy contributed to the load of a building. If no energy is used or required there is no contribution. Note: heat loss from a tank might contribute to the heat load in a positive way as to the cooling load in a negative way. In a solar hot water systems generally the “contributed” solar energy or BTUs would be measured with a flow-meter and 2 sensors (cold inlet and hot outlet of a solar storage tank, possibly a preheat tank). Solar fraction is the percentage of solar contribution to the total energy load (measured in water BTUs).
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.
Seasonal Efficiency is 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) of 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.