Renewables for hot water systems

Traditionally, renewables don’t mix with commercial hot water systems. Hot water for commercial projects must be stored at 65⁰c to prevent the risk of legionella. Yet with renewable systems the mantra is usually “the lower the temperature the higher the efficiency”. The high temperatures required by commercial hot water systems can therefore mean that renewables are unable to offer a suitable performance or return on investment. Whereas domestic systems and those for commercial properties that have a primary demand for heating can work with renewable because of the low temperatures required, this is not the case where the heaviest expenditure of energy is on hot water production, such as is the case for leisure centres and sports facilities. The demand for fast recovery times only exacerbates the problem.

But don’t abandon hope! With the right products, and a little inventive thinking, there are still some spectacular savings in running costs and carbon emissions that can be achieved using renewable products on commercial hot water systems. So today I’m going to take you through some of my favourite solutions.

Let’s start with small projects. A typical example is the sports club; often off the main gas grid, with a single phase electrical supply and a small budget for installation and renovation. The obvious solution here is a large store of water heated by an electric immersion heater. But Stiebel Eltron have a renewable option in the WWK 300 heat pump that could save 1/3 -1/4 of the running costs of an immersion heater, and which works on a single phase electrical supply. Because this specially designed heat pump heats to 65⁰C the water is sufficiently hot for safety. This pump also compares in price with a standard boiler and so makes an easily affordable option.

Larger projects – lets take the example of a sports or leisure centre – tend to have a gas supply and three phase electrical supplies and larger budgets, which both widens the options considerably and means that any solution must compete on cost with a mains gas boiler. Where a boiler is used I recommend considering our range of thermal stores - the SBS instantaneous water cylinders, which have a sealed store of water through which runs a heat exchanger. Run with a powerful fossil fuel boiler these offer very fast recovery times and are cheaper to run that immersion heaters: because the stored water is never in contact with the water that runs out of taps and showers it does not need to be heated to such high temperatures – saving on energy and money.

My first suggestion for a true renewable contribution to a commercial hot water system is a solar pre-feed. This is where solar thermal panels are configured to pre-heat water for high-temperature storage, reducing the energy required from a boiler to bring it up to temperature. This has the advantage that it is both great for new build and easy to bolt on to existing systems. The slight snag with this set up is that you can only harvest energy when it is hotter on the roof than in the tank – because water needs to be stored so hot for commercial use, this can destroy the yield from the panels. So it must be used in conjunction with the SBS thermal store discussed above to heat a pre-tank which then feeds the hot store. It is essential to keep an eye on the required flow rates for any showers (or rather the required rates for the thermostatic mixer valves that control the showers) being fed by this type of system. Too high a demand for flow and you lose some benefit as water will flow too fast through the pre-heat system to collect sufficient heat. Pre-feeds heated by heat pumps, using the same approach are also possible and can work really well where a pump is also being used for space heating.

A fully renewable system is also possible and here’s a solution that I have seen in operation in such contexts as a busy hairdressing salon or for up to 3 showers. It starts with a hot water heat pump with a store of hot water. This supply of water is connected at point of use to Stiebel Eltron instantaneous water heaters with bare wire technology. This unique technology means that the water heaters can accept pre-heated water and regulate their own response depending on whether the water is up to the required temperature. Early in the day when the stored of water is hot, it will run straight through them, without them turning on their heating element. If the tank is depleted and struggling to respond fast enough to return to temperature whilst demand is high at busy times, they will take on progressively more of the load.
There are of course also large projects which are off-grid, where a renewable solution is potentially highly cost efficient. The solution here is to use a method other than high temperatures to sterilize stored water. UV sterilization of the water store is the commonly used alternative in this situation.

So next time you are planning a commercial hot water system, take the time to think about whether there is a renewable solution, or a renewable element that can be included. You might be surprised!