Peter Broatch of Solex Energy outlines key commercial considerations and costs when choosing a solar thermal collector.
Most people are familiar with the workings of domestic solar heating systems, consisting of one or two panels or banks of tubes on a roof, heating a cylinder, but commercial systems are less widely known about or discussed.

The principals of operation of commercial solar heating systems are usually basically the same as domestic ones, as are the types of collecting panels available. The key differences are in scale, and the reasons for specifying solar, which tend to be driven more by financial considerations - ie payback time.

There are many commercial operations which have very high annual heat demands. These needs are usually served by gas, which is a relatively cheap fuel, so any renewable replacement has to be financially very competitive.

When the various different renewable heating technologies
are examined, it is found that really the only viable options are biomass and solar thermal. Solar PV (electricity) is an expensive option for providing heat. And, although wind energy could be used, it is generally more economical to sell any form of electrical energy back to the grid. Heat pumps are generally found to be expensive in both money and carbon terms when all the
calculations are worked through.

The main commercial application for solar thermal is for generating hot water in industry sectors with high usage, such as food processing, dairies, hospitals, care homes, hotels, caravan sites, schools etc. Other useful applications for solar heating include industrial process heat and heating swimming pools.

Space heating can be derived from solar thermal, although most commercial buildings tend to have high heat demands and low thermal mass, so the heat storage available to even out daily fluctuations in solar heat supply/demand needs to be carefully examined.

Solar thermal collectors generally fall into one of four categories, listed here in descending order of efficiency per square metre, and also descending cost per square metre:

Evacuated tubes

These are glass tubes of about 8cm by 2m installed in banks. These are efficient per square metre when generating very hot water. Their chief disadvantages are in cost and fragility. Commercial installations of evacuated tubes typically consist of banks of tubes mounted in metal frames, with the frames mounted on a pitched or flat roof. Typical cost £250-400/m2

Flat panels

Aluminium framed glass panels are typically around 2-3m2 each. Flat panels are now close in performance to tubes. They are less fragile, and are preferred in the bigger solar markets such as in Germany and Greece. For commercial applications, the mounting arrangements are very similar to those for tubes. Typical cost £200-270/m2.

Solar roofing

Solar roofing is a transparent roofing material with integral solar absorber. It is a relative newcomer to the market. It has the advantages of durability and lower square metre cost, and lends itself to large scale whole-roof installations. This roofing product may consist of glass tiles or slates for the more traditional pitch roof installations, or lightweight solar cladding for metal profile roofs. Typical cost £90-160/m2 (offsetting roof material costs takes this to £70-150/m2).




Unglazed

This involves extruded plastic tubes or mats with numerous water ways. Their main applications are for low-temperature heating such as for swimming pools, and for commercial applications it is usually laid on a flat or low pitched roof. Typical cost £50-70/m2. (Costs are for the solar collector and mountings only, excluding installation)

If the design brief is to heat a relatively small amount of water to a high temperature, then this may be achieved by using flat panels or tubes. Where large quantities of water are required to be heated or pre-heated, and in the case of pool heating, the most important factor is how many square metres you get on the roof for your money. In these cases, solar roofing or unglazed collectors will usually win through on cost.

After the solar collectors, the next most expensive part of the system will usually be the hot-water storage. Suitable large hot-water storage vessels ranging in size up to 5,000 litres are now available from a number of companies, costing in the region of 75 pence to £2
per stored litre.

The water storage vessels should be sized correctly: too small and
the solar collectors will be under-utilised; too large and costs will be increased unnecessarily. The volume of hot water storage required relative to the solar collecting area will be determined by the pattern of water use.

Premises such as sports centres and schools will need a relatively larger storage volume to collect heat over weekends and other non-use periods, to make best use of the area of solar collector installed.

At other end of the scale are plants with a continuous need for hot water, where the solar pre-heats incoming cold water for top up using another heat source. Here,
the solar heated water is often used practically as quickly as it is produced, so the hot-water storage may be minimal or even non-existent.

Swimming pool systems store the heat in the pool, so no other hot-water storage is required for these systems unless hot water for showers is required as well. Pool systems typically use an inline tube heat exchanger to input the heat into the pumped pool circuit ahead of the boiler heat exchanger, although some solar roofing and unglazed systems pass the pool water directly through the solar collector.

As can be seen, the specification process needs to look at both the annual energy requirements of the business and the pattern of heat use.

Different systems and scenarios then need to be costed out using different types of solar collector field and sizes of water storage, so that the best energy payback versus cost can be achieved. There are some professional solar software packages on the market (e.g. TSol or Polysun 4, ranging £300-£800) which may be worth investing in. Alternatively, a good energy consultant can calculate the energy performance of different systems.

Solex Energy 01305 837223 www.solexenergy.co.uk