Objectif
The project covered the use of evacuated tube type solar collectors for the heating of water for use by the canteen on the GEC Whetstone industrial site near Leicester in the UK. It was intended that the solar water heating facility would substitute for the site boiler/distribution system during the summer months when this system gave poor efficiency.
Investment cost : 139,400 UKL for the system study based on a scheme of 50 m2 collector area.
The Whetstone advanced collector solar water heating scheme has been successfully installed and has operated in the planned mode over the lifetime of the project with only minor difficulties.
The measured system data and its subsequent analysis has yielded detailed information on the performance of the system under real operating conditions. The main findings are:
i) The system operates with a yearly average efficiency of 38%
ii) An overall solar fraction of 24% of the system demand is achieved. The
system output closely matches that predicted.
iii) The MEAN daily hot water demand is generally constant over the year apart
from special periods such as holidays.
iv) The daily hot water demand has a good match to the solar radiation profile.
v) Pre-heat tank temperatures in excess of 70 deg. C were recorded during
summer weekend periods.
vi) The detailed monitoring exercise identified significant energy losses in
peripheral items within the standard canteenwater system such as
dishwashers. Corrections of these losses achieved associated savings
in hot water energy costs of around 10%.
Taking the cost of 21700 UKL for 50 m2 of collectors, annual operating cost of 300 UKL, energy delivered 19500 Kwh/yr the amortized energy price over 20 years is 11.6 p/kwh at real interest rate of 6.5%. Current energy costs are 2 p/kwh.
To summarise, the existing retrofit scheme cannot be considered an economic proposition against present day site energy costs. The incorporation of advanced solar heating systems into new buildings, however, presents a more realistic prospect. In this instance, whith imaginative design, there is little reason for the system costs to be more than those of the collector array. For example, storage tanks are already required and it is a simple matter to specify tank(s) suitable for storage of pre-heated water with minimal extra cost. In addition, support systems for the collector array can be adapted from thebuilding structure.
Reduction of actual collector costs would, of course, have a direct effect.
Regarding future market potential, a number of areas have been identified as significant or requiring attention. These may be summarised as follows:
i) Further improve collector design and/or production techniques to give
lower collector cost while still maintaining the performance advantage.
ii) Explore methods to minimize the use of absorber tubes in the collector
through, for example, the use of a diffuse back reflector, again reducing
costs.
iii) Avoid retrofit schemes unless they can be approached on a "modular
package" basis, instead incorporate systems in new buildings allowing use
of existing building/system components in a dual role.
iv) Concentrate first on areas with a combination of high average insolation
levels, high conventional energy costs, and the necessary economic
resources.
SYSTEM SIZING/LAYOUT
As the starting point in the system sizing procedure the accepted figure of 50 litres of pre-heat storage capacity per m2 of collector absorber area was taken to size the pre-heat tank capacity at 2500 litres. Checks on the canteen water demand confirmed peak usage around the midday period attributable to the use of an automatic dishwater. Average daily demand was some 5000 litres/day. Hot water usage was for five days only due to canteen shut down over the weekend..
To accomodate the situation where the pre-heat tank could reach temperatures above the system requirement of 60 deg. C under weekend demand conditions with high insolation, either a large capacity tank or a flow mixing arrangement had to be selected. Of these options, a mixing valve was chosen as preferable since a large capacity tank would have involved extra load bearing supports and consequent increased installation costs.
Following on form the initial planning application, specific planning and building regulations approval were sought as some minor alterations to the building were necessary in order to support the collector array. The collector support system was designed in accordance with expected wind load conditions and consisted of a lightweight interlocking framework attached to rolled steel joists positioned across the exterior supporting walls of the building.
The Thermomax type THS200/A collector was chosen for the project.
Due to the previously tendency for the solar pre-heat tank to experience high temperature during weekend/high insolation conditions, careful consideration had to be given to the specification of this tank. A conventional galvanised mild steel tank, when connected to copper pipework, is not suitable for the storage of water at high temperatures due to galvanic corrosion. Stainless steel was considered but costs proved excessive. A cheaper alternative based upon a polypropylene/glass fibre laminate was eventually selected allowing the tank to be fabricated to the required dimensions. Maximum operating temperatures was specified as 95 deg. C.
Conventional copper pipework, insulated to a high standard, was specified for all interconnection.
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Coordinateur
LE8 3LH WHETSTONE
Royaume-Uni
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