Water Heating Basics

• 1kWh of heat energy can heat about 30 Litres / 7.5 US gallons of water up from cold to showering temperature.

• A household of 5 people will use between 10-15kWh of energy each day to heat water (not including space heating).

• In the winter more energy is needed to heat water as the cold water is colder, and people often have longer and hotter showers than in the summer.

• Water is normally heated by electricity or gas, but solar can be used to offset between 60-80% of this energy.

Storing Hot Water

The most common hot water system involves heating and then storing hot water in a tank. When hot water is used, it is drawn from the top of the tank, where the water is the hottest, and fresh cold water is delivered into the bottom of the tank. Because hotter water is lighter and sits above colder water, it is realistic to see a tank that has bottom half cold and the top half hot... this is referred to as thermal layering, or stratification. Eventually after enough hot water usage the water suddenly turns cold, which is something everybody has probably experienced! 

Tanks are designed to enhance thermal layering to maximise hot water supply by reducing turbulence when the cold water enters the tank.

Solar Tank Design

The performance of a solar collector is highest when heating cold water. As the temperature of the water increases the efficiency of the solar collection will drop, the amount of performance loss depending greatly on the type and quality of the collector. For this reason thermal layering is important in a solar storage tank, ensuring that solar is always heating the coldest water possible.

Given that the coldest water is in the bottom of the tank, the solar heating is always in the bottom area, either direct flow or with a coil heat exchanger.

Ideally a dedicated solar tank should be used which then feeds a primary tank with solar pre-heated water. This format is ideal when retrofitting to an existing system as does not require any major changes to the plumbing layout. The cold supply is fed into the solar tank, and the solar tank outlet feeds the cold inlet of the main tank. Having a dedicated volume of water for the solar to heat eliminates competition from the existing energy source, which would happen if the tank was also heated by electricity, gas etc (unless timer or manual boosting control is used).



How Solar Collectors Heat Water

The question is often asked “How much hot water will I get”, or “What temperature is the water from the solar collector”. Neither of these questions are possible to answer directly, as some additional information is needed.

Firstly, in most applications, solar does not heat the water in a single pass like a tankless gas heater would. The solar collector heats the water gradually throughout the day, normally by about 10^oC / 18^oF each pass. The flow rate through the collector is also quite slow, around 3L/min / 0.8gpm for an average domestic sized system.