Solar hot water simplified

Rinnai, solar thermal, DHW, renewable energy
Evacuated-tube solar collectors are an effective way of harvesting solar energy.

Using solar energy to help meet a building’s requirements for domestic hot water delivers benefits throughout the year. The key issues are how to make up the shortfall and address safety issues. Chris Goggin of Rinnai looks at one approach.

We need to find ways of using current renewables technology — be it solar-thermal, solar-PV or heat-pump technology — in a practical manner maximised by traditional fuel sources. One of the most ecological and economical methods is to design a system with primary renewable sources and supported by secondary sources such as continuous-flow hot-water heating.

Products in this category include a thermal store of water heated by solar energy and both flat-plate and evacuated-tube collectors, each with specific benefits to suit the differing design aspects and building needs. Either option will provide years of low-maintenance, low-cost energy for heating water during the summer months — and beyond.

Water heated by the solar panels is unlikely to come up to a workable temperature all year round, so the boosting the temperature of the pre-heated water to the required temperature by a secondary heat source will be a requirement. Such a system will maximise the benefit of solar energy and optimise gas input.

In this scenario a continuous-flow water-heater can be utilised as the secondary heat source to give the solar-heated water the required boost.

Modulating continuous-flow water heaters are well suited to this type of application. Rinnai units, for example, can modulate from 56 kW at full output down to 2.27 kW, so they use only as much gas as is needed to bring the required volume of water to temperature.

For example, if water to the water heater has already been preheated to 40°C by solar energy and a temperature of 60°Ç is required with a flow rate of 15 l/min, a Rinnai unit will increase the temperature by the required 20 K (±1K) whilst only using 21 kW. If the demand drops to, say, 5 l/min the heater will modulate down accordingly to an output of 7 kW.

Fossil fuel is only used when incoming water pre-heated by solar energy is below the required set-point.

Rinnai, solar thermal, DHW, renewable energy
Using a solar-thermal store to preheat domestic hot water before it is brought up to the required temperature by continuous-flow water heaters optimises the balance between the use of renewable energy and fossil fuel.

Another benefit of using a continuous flow water heat as a booster is that you can wipe out any worries about legionella being a problem with stored water.

Hot water will always be delivered to the building at the required minimum of 60°C, so the system will always comply with Building Regulation L8. If a secondary return system is used, there is a guarantee that the return will always be at 50°C — again in compliance of L8.

Both flat-plate and evacuated-tube collectors work best when facing due south. However, evacuated-tube collectors will outperform their flat plate counterparts in situations where only east/west facing options are available.

Evacuated-tube collectors have multiple glass tubes that heat up by absorbing energy from the Sun. The vacuum-insulated tube retains most of this heat, and the constant profile of the round tube means that the collector is always perpendicular to the Sun’s rays. The energy absorbed is therefore fairly constant over the course of the day.

In the summer months, the performance of the flat-plate option will be close to that of an evacuated tube unit when facing south. However, evacuated-tube collectors have better overall results in the early and late seasons and are not affected by adverse weather conditions, producing impressive temperatures even on cloudy days. However, flat-plate collectors are perhaps more suited to vulnerable locations because of their exceptionally sturdy and durable build.

The pairing of continuous-flow water heating with renewables technology is the perfect fit for a never-ending supply of domestic hot water.

Chris Goggin is associate director of Rinnai UK.

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