Solar energy meets instantaneous hot water

Stokvis, DHW, Solar thermal
Plate heat exchangers can be combined with solar thermal energy to produce domestic hot water without the problems associated with storage.

Paul Sands describes an approach to producing domestic hot water using solar energy that sidesteps the problems of storage.

The latest solar-thermal water-heating systems are highly efficient, with the ‘free’ solar energy received by the Earth in just 30 minutes equivalent to the energy used by the entire human population in one year.

Even in the UK’s less than sundrenched climate, heat from the Sun remains an attractive and viable option. Indeed, on a sunny day, parts of the UK experience levels of solar energy equal to 60% of those at the equator.

Even free energy ought to be used efficiently, and we use an advanced form of evacuated tube solar collector called Ecotube which is designed to maximise the solar energy absorbed and minimise heat loss through radiation.

This system has substantially reduced the amount of ‘parasitic’ energy lost by most evacuated-tube systems (energy used by the system itself during the process of converting solar energy to heat). As a result, it requires smaller pumps to circulate the fluid in the evacuated tube collector, which means that there are fewer drops in pressure through the system — giving a sustained level of heat.

These sorts of advantages, combined with the fact that it is a sustainable heating solution, makes solar thermal heating the most realistic renewable-energy solution for most new and refurbishment applications.

However, there is a potential hitch. Solar systems require some form of storage, but this is not necessarily the most efficient way to deliver hot water when the primary heat source is from a boiler.

Conventional wisdom dictates that a coil placed within the solar vessel is used to transfer heat from the solar panels, with a second coil within that same solar vessel fed from a boiler to supplement the solar heating.

This approach, of course, implies the need for hot-water storage with indirect heating from a boiler for hot water to be immediately available. Even with the best insulated solar vessel, this water storage will lose heat — resulting in the boiler cycling on and off to maintain the temperature of the stored water, even when water is not required.

Our answer to this is a sort of ‘hybrid’ water-heating system employing solar-thermal technology. Water in a storage vessel that has been indirectly heated by solar energy has its temperature topped by as required by a boiler before passing to a low-water-content plate heat exchanger to provide instantaneous hot water — without the need for storage.

Solar hot-water systems take advantage of the ‘free’ energy supplied by the Sun. And, although not renewable systems in themselves, plate heat exchangers offer a host of benefits. Econoplate models (from Stokvis), for example, are ideal for use in systems with rapidly changing heat outputs because they have a low water content and can therefore respond to changing requirements quickly.

They come complete with primary pump(s), a fast-acting motorised control valve on the primary circuit and a purpose-built PID controller to sense secondary-water temperature and control the motorised valve.

They can also be accommodated into most building-management systems via volt-free contacts and offer the controlled delivery of hot water over a wide range of demand requirements.

A ‘marriage’ of these two exceptional systems — solar and plate heat exchanger — minimises the use of fossil fuels and results in a highly efficient, cost effective and sustainable solution for hot-water generation for all commercial applications.

This system ensures peak efficiency from the boiler because it involves heating the water instantaneously through the plate heat exchanger on a modulated control valve without feeding into any kind of storage.

Stokvis, DHW, Solar thermal
Substantially higher temperatures are available from Ecotube evacuated-tube solar collectors than from flat-plate types.

This design also has a second big potential advantage. The risk of legionella is increased where water storage takes place, so these systems would need to run an anti-legionella pasteurisation cycle, raising the water temperature above 60°C for a certain period of time to kill off any bacteria.

Although it would still be recommended to run an anti-legionella cycle with solar-storage/plate systems, The potential possibility of legionella breaking out into the system is greatly reduced since the water is raised instantaneously by the plate to 60°C, provided the plate is sized to deal with the full load instantaneously. There is, therefore, a greater likelihood that the water will be at a high enough temperature to kill off any legionella.

The proof of the pudding is, of course, in the eating. The question is how well a system combining solar thermal and a plate heat exchanger works in practice? The answer is very well. We have already successfully installed this arrangement on several projects and, as more designers become familiar with its significant advantages over the alternatives, I would expect take-up to grow.

The complete solar panel and heat exchanger system is remarkably efficient.

On a sunny day, a single Ecotube panel can match the performance of an average domestic immersion heater, Winter or Summer.

The conventional heating system (oil, gas or immersion heater) will use far less energy, if any, to raise the water temperature to the level set on the thermostat.

The less conventional energy used, the smaller the energy bills will be — savings that will increase as the price of energy rises over the lifetime of the panel.

Because of their sophisticated vacuum-tube design, Ecotube panels are air temperature independent, so they work extremely well on cold, clear, frosty days.

The indirect system used by the Ecotube eliminates problems caused by the build-up of limescale, and also ensures that the system cannot freeze in winter. Glance at the simple chart on this page, and you will see just how effectively the Ecotube raises the temperature of the mains water supply throughout the year.




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