Solar Water heater Tank Sizing

Heat Radiated by the sun is the most abundant source of energy on earth, it's available for free and converting it to other forms of energy such as hot fluid of electricity has never been cheaper than before. Some places on earth have 4000 hours of yearly sun exposure (such as Arizona and Egypt) and other places have only 1500 hours of yearly sun exposure (such as St-John's in Newfoundland, Vancouver in British Colombia or Sherbrooke in Quebec). In colder climates sun hours are more frequent in summer (when it's less needed) and less available in winter (when it's mostly needed).

This is the case of most major Canadian Cities from east to west coast, making a properly sized thermal energy storage tank a necessity for storing captured heat when heating demand is relatively low and using it later on when heating demand is relatively high. Thermal Heat Storage in this case has two purposes: The first is to eliminate any wasted heat (by storing excess heat) and the second is to be able to use already stored excess heat in the absence of panels' sun exposure.

As an example, an over-sized solar water heater tank that only allow a water temperature increase of less than 10⁰F is simply useless.

SOLAR HEAT INTENSITY IS LOCATION DEPENDENT

Sun's thermal intensity isn't the same everywhere in Canada (3). Solar insolation is at its peak in the southern part of Alberta, Manitoba and Saskatchewan as seen in Canada's solar radiation map.
Higher Solar Radiations intensity, expressed in Watt by Square Meter of Solar Collector, requires more thermal storage volume. Consequently less Solar Radiations Intensity require less thermal storage volume.
It's highly recommended for solar designer or installer to avoid using universal rules of thumb (Like 2 US Gallons of Thermal Storage Volume for every 1 Square Foot of Vacuum Tubes Solar Water Heating Collector). What's good for Calgary might not be good for Sherbrooke or St-John's and vice versa.

CALCULATING SOLAR WATER STORAGE TANK

Calculating Solar Water Thermal Storage Tank Volume will be done using the second law of thermodynamics. This Simple Law States that Heat Collected by solar water heating collectors will be equal to the rate of change of storage tank's water internal energy divided by the Sunshine exposure time.

Water Density (Kg/m³) x Water Volume (m³) x ΔU (KJ/Kg) / Δt (Second) = Solar Heat (KW = KJ/second).

Water ΔU (KJ/Kg) = 4.18 (T2 - T1) (°C).

The above formulas are tabulated for both winter and summer time assuming the followings:

1. Winter Sunshine (Watt/m²) corresponds to first of February of every year. 
   
2. Summer Sunshine (Watt/m²) corresponds to first of July of every year. 
   
3. Storage Tank Initial temperature is assumed to be 20 (°C). 
   
4. Thermal Efficiency of Vacuum Tube Solar Collectors of 40%. 
   
5. Solar Collectors are perpendicular to sun's radiation (which is not always true). it's a conservative assumption. 
   

CONCLUSION

Solar Thermal Storage Volume by unit area of solar water heater collectors varies from Canadian City to another. it can be as low as 0.59 US Gal./ft² to as high as 1.06 US Gal./ft² which might have a big impact on the cost of your solar water heating project. The Storage Tank represent a third of your total material cost and is the central piece that connects your solar water heating installation to the rest of your house or building.

N.B: The above table was prepared based on the above mentioned assumptions. Changing one of the assumptions will make the above tabulated results obsolete from a technical standpoint. For further details please contact our technical team or simply call +1 (888) 686 7652.