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March 7 2018

Taking Storage Out Of The Firing Line

Paul Sands of Stokvis Energy Systems examines how solar thermal hot water can best be utilised within the food industry.

All life on Earth is dependent on the energy we receive from the star at the centre of our solar system; yet there is still more benefit to be derived from this vast fusion reactor.

In Victorian times, industrialists built factory roofs which featured ‘north-lights’ to provide optimum amounts of free illumination while today’s architectural designs help most types of building benefit from passive solar gain.

In the domestic environment, solar collectors are normally linked via a pump-station, to a twin-coil cylinder which will also be fed from a conventional boiler or, perhaps, a heat-pump. This arrangement is perfectly satisfactory in many ways and can provide the average household with the vast majority of its hot water needs during the year.

When looking at commercial applications for food production or food processing plants, the dynamics of demand are very different, and the way the solar energy  inputs into the equation needs to be reconsidered.

Production facilities are likely to be operating over long working hours, possibly with a two- or three-shift system. This inevitably means using very large amounts of hot water at medium to high temperature, for washing vegetables, animal carcasses and equipment, or actually mixing it into processed foods. 

Again, solar panels will only be able to meet part of the demand, although the way the system is designed and set up can make a big difference to efficiencies; and consequently fuel or cost savings.

Commercial premises will almost certainly be equipped with modern boilers running on gas, or some other form of fossil fuel, to offer reliability of both heating and hot water. Yet, having even the latest modulating gas boilers firing into large storage vessels as a means of providing process water does not present the optimum answer.

Even well-insulated storage vessels will lose heat whether water is being drawn off or not, causing the boiler to cycle on and off, as the temperature drops. Having solar coils connected into the system here will not alter the situation. Fuel will still be wasted in maintaining the temperature while there is also a need to provide a boost to kill the Legionella bacteria on a regular basis.

It is far more efficient to have an array of solar panels storing the free energy collected within what serves as a preheat storage vessel. This way, incoming potable water from the mains or a borehole can be raised from around 10? what? to the temperature required, or if the solar gains are not adequate, some temperature in between.  Then when the temperature of the water requires raising to the required set point, the boiler in conjunction with a  plate heat exchanger (PHE) achieves this efficiently and instantaneously.

What is more, because both modern modulating gas boilers and the PHE units feature low water content, there is very little thermal inertia and energy efficiency is therefore optimised.

A further benefit of not storing the water to feed around a plant is that it almost completely eliminates the risk of Legionella contamination. This is because the instantaneous temperature rise through the plate heat exchanger will kill any bacteria and the need to regularly raise the temperature of a store to above 60 degrees centigrade for a prescribed period is eliminated.

The type of evacuated tube solar collector supplied by Stokvis is very simple to orientate for optimum performance, irrespective of the roof configuration. This offers an energy yield far in excess of the 528 kW/hr per square metre normally taken as an industry standard. In fact, it is 61% higher at 850 kW/hr over an average year. The installation can then be designed to fit the space available and the demand.

Enough solar energy reaches the surface of the earth every half hour to match mankind’s consumption for a whole year. It all comes down to the economic realities of harnessing the solar energy and, by combining a solar thermal system with plate heat exchangers to supply hot water to the food industry, it is one of the more efficient means of doing so.

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