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How solar hot water works

Luke Boyden

Luke Boyden

Overview

The energy received from the sun can be used to heat water for a building, known as solar hot water. That hot water may then be used for heating of a building, energy storage, or more commonly as a hot water source. If the sun doesn’t shine for long enough, or intensely enough, to heat the water to the desired temperature, an additional booster heater can top-up the water to the required temperature.

The system works as follows:

 The selection and design of a solar hot water system is determined by:

1.     Insolation (irradiation) – solar energy received for a given area and time

2.  Seasonal environmental temperature range – average min and max temperatures over a year

3.     Daily temperature range – average min and max temperatures over a day

4.     Hot water requirement in winter

Solar hot water systems used for providing hot water can be classified in various ways.

System type – direct and indirect (open and closed loop), passive and active

Solar hot water systems either heat the water to be used (direct), or heat a working fluid and transfer the heat via a heat exchanger to the water (indirect). An indirect system would use a working fluid consisting of water and anti-freeze, and possibly corrosion inhibitors.

Indirect systems offer better protection from temperature extremes (freezing and overheating), though will be more expensive and require slightly more maintenance (working fluid changes).

A system is passive or active, depending on whether it contains a pump to move the working fluid (water or otherwise).

Active systems use a pump to transfer the fluid from the collector plates to the water tank. They are generally less visually intrusive, can have better efficiency as tanks can be stored in warmer areas, though require more maintenance and are of higher cost.

Passive systems rely upon thermosiphoning (a method of passive heat exchange based on the physical property of convection - hot fluid rises through cold fluid). This simplifies the system mainly by eliminating the need for a mechanical pump. Consequently, the tank must sit above the collectors, often on the roof.  

Collector types – flat-plate and evacuated tubes

The two main types of collectors are flat-plate boxes and evacuated tubes.

Flat-plate collectors are sealed boxes with a transparent front and a series of tubes inside to absorb energy and conduct heat to the fluid. They tend to be cheaper than evacuated tubes as they have simpler construction, but are sensitive to angle of inclination and the ambient temperature – they are less well suited to colder environments as they suffer greater heat loss, and so reduced efficiency.

Evacuated tubes are transparent tubular collectors that have a vacuum between the transparent exterior and interior pipework. This insulates the working fluid and improves the efficiency of the system, which is especially useful in cooler climates. They tend to be lighter but more expensive.  

In very sunny and warm conditions, where heat loss is less of an issue than in cooler climes, flat-plate collectors can offer a low cost solution. Whereas in cooler locations getting a more efficient evacuated tube system is likely to be money well spent. 

Boosting systems

Sometimes additional hot water may be required because of either increased usage of hot water, or less favourable weather conditions not supplying anticipated energy needs. To ensure that water received at the tap is hot enough, solar hot water systems can be boosted with an additional heater, normally fuelled either through solid fuel, electricity, or gas.

The energy used to boost could originate from renewable sources, either supplied from a green electricity company, or provided from a domestic install. However, if there is limited volume of hot water available due to weather conditions, it is less likely domestic photovoltaic renewable energy systems which also rely on favourable weather conditions, would be able to supply the additional demand or shortfall.

Boosters can either be manually controlled or automatically operated. If a continuous flow water heater system is in use, where the water is heated as it is used, the booster will supply enough energy to heat the water temperature to the required output. If a storage tank system is in use, the booster will heat the water stored in the tank and maintain the required temperature.

Consideration of usage habits (in order to properly size a tank) and a timer system should be employed to minimise the boosting needed and additional energy used. For example, if only a thermostat (and no timer) control is used with a storage tank system, the booster may heat the water all through the night in order to maintain the desired water temperature, wasting energy when hot water is not required. 

Implications for your carbon footprint

Boosting your solar hot water system with another renewable source like a heat pump or solar power system is ideal for reducing your carbon footprint. However, if only a small amount of boosting is needed it may be best to use the gas or electricity system you already have, rather than install another booster, because of the embodied energy of a new booster system.

Positioning and structural considerations

Systems should be installed in a position that maximises sunlight exposure on the collectors (like all solar powered devices). To maximise performance collectors should be installed at a specific angle to the horizontal, which can be calculated from the install location. The collectors and some tanks will generally be mounted on the roof of the building, as this is the place that is least overshadowed. Most roofs will be able to support this additional loading, but if your system installer does not advise on this be sure to consult a structural engineer.

Maintenance considerations

Collectors will accumulate limescale in hard water areas unless a water-softening feature is incorporated.  Freeze protection should be included in any area where near freezing temperatures are experienced. Indirect systems will need their working fluid changed on a maintenance schedule.