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Plate Heat Exchangers Reducing Storage in Solar/Lpg Water Heating

Plate heat exchanger technology powered by lp gas boilers can be combined with renewable solar thermal systems to produce a highly efficient water heating solution that avoids the need for excessive water storage, says Paul Sands of Stokvis Energy Systems.

The ‘free’ solar energy received by the earth in just 30 minutes is 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 energy option. Indeed, on a sunny day, parts of the UK experience levels of solar energy equal to 60 per cent of those at the equator.

The latest solar thermal water heating systems are highly efficient, but there is a potential hitch – solar systems require some form of water storage, but this is not necessarily the most efficient way to deliver hot water when the primary heat source is from an lpg gas 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 fed from an lp gas boiler placed within that same solar vessel to supplement the solar heating.

This, of course, implies the need for hot water storage with indirect heating from an lpg boiler, since this arrangement is unable to supply hot water instantaneously. Even with the best insulated solar vessel, this water storage is likely to lead to a loss of heat, resulting in the lpg gas boiler cycling on and off even when water is not required.

Our answer to this is a sort of hybrid water heating system employing solar thermal technology, with all the benefits this brings, combined with an lp gas powered boiler and low water content plate heat exchanger, which can provide hot water instantaneously, topping up the water temperature as appropriate 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. Stokvis Econoplate models, for example, are ideal for use in systems with rapidly changing heat outputs because they of are low water content and can therefore respond to changing requirements quickly.

They come complete with primary pumps, 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 – results in a highly efficient, cost effective and sustainable solution for hot water generation. This system ensures peak efficiency from the lpg 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.

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 60ºC for a certain period of time to kill off any of the bacteria.

Although it would still be recommended to run an anti-legionella cycle with solar-store/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 60ºC, 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 does a combined solar thermal/plate heat exchanger system work 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.

Contact Stokvis Energy Systems on 0208 783 3050 www.stokvisboilers.com

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