Revue de reunion

BHRA Conference Oxford, UK, 25-27 March 1987

The conference held at Oxford University in March this year was the third in the series organized by the British Hydromechanical Research Association (BHRA) of Cranfield, relating to large-scale applications of heat pumps. Essentially, domestic uses and window-insert units were excluded. This international event attracted 32 papers and 121 delegates from 21 countries as far apart as India, Iceland and New Zealand. Three delegates represented the USA.

Six subjects were divided into nine sessions, one of which consisted of poster presentations. District and building heating, process applications, compressors and controls, absorption systems and a forecast session on future developments were covered in the proceedings.

Energy engineers, technologists and planners have for a long time considered contributions from heat pumps as ranking with wave and wind power, tidal and geothermal techniqes, in that sufficiently intensive research should prove economic and significant. Well, the current answer to that expectation is disappointing: economic operation cannot be achieved and, if pay-back time is the crucial factor, then five to 10 years is all that can be expected.

This puts large-scale heat pumps into much the same unproven category as wave power and, as with geothermal energy, research and development continues in a less than intensive fashion. Economics may be the determinant for most applications but like a number of other developments which started by finding niches that became specialist and exclusive, the roles now being developed could well be commercially rewarding for other reasons. The opinions on which the current verdict is based were culled from commercial and consultant sources. Academics and research engineers are obviously sufficiently encouraged by their (so far uncommercial) results as to continue. The fate of some officially supported development projects may well, however, be lack of replication.

Two papers surveying group results produced most of the commercial significance. The Swedish National Testing Institute has been performance testing some 26 large units of 10-30 MW heat output. All of them feed municipal district heating using sewage, lakes, industrial waste water or geothermal sources. The reason for this perhaps surprisingly large activity was not made plain at Oxford and only became obvious when a representative of the Swedish State Energy Commission underlined the consequences of the decision to demonlish all nuclear energy plants by AD 2010. This was at the Reading University conference on energy options two weeks later. Sweden is in a hurry to provide an alternative for the nuclear 50% of its electricity supply.

This performance testing set results against guarantees provided by the contractors. The conclusion: ' . . . on

average, performance compares well with guaranteed values'. The message from the paper came clearly and unequivocally and was confirmed by every other forthright report: over-sizing leading to less than full-load, full time working is asking for disaster. No more contingency allowances in design can be permitted in heat pump practice. If the COP is to come out as planned then all parameters have to be precise. Equally clear, if not so expressed, was the message that engineering machine design can be no more exacting than in adapting refrigeration principles to the purpose of heat transformers. This Swedish experience spoke of reliable performance with one major problem, that of refrigerant leakages.

Reviewing the seven-year performance of five industrial heat pumps in the UK supported by the Department of Energy's Energy Efficiency demonstration scheme, ETSU at Harwell reported: 'None of the projects achieved the projected savings'. Despite this they say: ' . . . there is still a large potential to be tapped'. Again, the researchers say: ' . . . gross oversizing occurred'. Also, as some compensation: ' . . . design lessons have now been learned and there is no reason why we need to learn such lessons again'.

The five industries involved in these conclusions are food processing (steam turbine drive), UHT milk sterlizing processing (electric motor), textile finishing (gas engine) and food drying (gas engine) (two examples of the last). The compressors varied from reciprocating to single screw and the heat sources from 20 to 53°C with designed COP between 4 and 6.7. Operating hours ran from 4000 to 8000. The worst payback proved to be 10 years instead of the projected 2.5.

Compressor technology in heat pump design is reasonably well understood; the use of the absorption cycle with its need for high grade heat as the generator drive is another matter. No less than seven papers in two sessions described both practical working systems and research in progress. Among the latter was the intriguing 'Solid-gas chemical heat pump in the range 150-500°C ' from three members of the French Institut de Science et Genie des Materiaux et Procedes, Perpignan. A prototype for 20 kW has been constructed.

Such high temperature working must have confused the EEC Commission from Brussels who reported on 22 research and development projects in Europe and spoke in generalities: 'The use of heat pumps for industrial process heat is limited to applications which use temperatures below 120°C. This limits the potential market to ~ 10% of the overall industrial process heat consumption. The potential energy savings lie at ~ 1% of the overall primary energy consumption'. Engineering academics, whose activities often demonstrate a search

0140-7007/87/050307~)2503.00 © 1987 Butterworth & Co (Publishers) Ltd and IIR Rev. Int. Froid 1987 Vol 10 Septembre 307

Conference review

for novel areas of research, rather than commercial or industrial need, seeking solutions to practical problems, are indeed venturing deeper and deeper into the very field Mr Zegers of the European Commission has in mind. This is true of the Japanese in particular. Aiming at a COP of 8 for the production of water at 85°C they are developing a four-stage cascade compressor system using a mixture of non-azeotropic freons. They are also working on heat storage systems using gas clathrates and other chemical reaction systems.

The use of water/steam as a refrigerant is quoted by the Japanese and others now involved in high temperature work. Related research and development continues in India at the National Chemical Laboratory, Pune, where mixed working fluids have been a feature of the work - with consistently higher CO P than for R 11. Discussion of the highest CO P mentioned at this conference centred on a BP refinery at Gothenburg, Sweden, where pinch technology and low pressure steam at 4.5 bar with an evaporating temperature of 70-80°C, condensing at 150°C, produce a COP of 10 and a pay-back time of two years. The reader really must consider the full paper if this is to be taken in context.

Pursuing the water refrigerant theme, the West German Siemens laboratory has developed a high- temperature heat pump with an entirely novel form of compressor. Eschewing reciprocating, centrifugal or Roots-type machines, Siemens has adopted the liquid- ring principle used more specifically for sub-atmospheric gas pumping. For use with steam compression a thermal oil is employed; condensation temperatures up to 120°C and evaporation of 90°C were attained with a single-stage compressor. Test running has exceeded 150 h and lifetime testing is planned.

Moving from the large to the small, the Cryogenics Group of the Engineering Science Department of Oxford University has developed moving-coil linear-action compressors mainly intended to cool infrared sensors in aerospace applications. It is intended, however, to develop these essentially oil-free devices into air-to-water heat pumps which the sponsors believe could lead to cheap equipment for steam raising and hot water heating systems. Initially, cost limitations are recognized - in the permament magnets of the load-speaker type movements, spring suspension and fine tolerances.

Steam compression forms the basis of co-operative research in France, conducted by Electricit~ de France and Alsthom-Rateau, who have completed and tested a two-stage centrifugal vapour compressor working at 50 000 rev min- 1. Work will be continued by EDF, with the testing of a new system of desuperheating before the first industrial pilot plant is built.

What you do with upgraded heat is, of course, the crucial test of any heat pump system. District heating, or the space heating of sufficiently large building complexes is one aim of much research based on the availability of large volumes of low temperature water and wastes. In Spain, Cobra SA has developed plant for year-round air conditioning and heating of 20000m a of mixed accommodation in five adjoining buildings in an area close to Madrid. A solar-assisted heat pump is featured.

This unit produces 1600 x 106 kcal of heat, using 15 ~o from direct solar heating, 35 Y/o from solar upgrading by the heat pump, 25 ~o from off-peak electricity and 20~o from the electricity-driven compressor. A further 5~o comes from daytime electricity.

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District heating using three 100kW heat pumps at Vallentuna, 16 km north of Stockholm, serves a total of 150 apartments with heat derived from eight 'geothermal' bore holes, each of 114 mm diameter, inclined at 20 ° from the vertical. The original intention was to 'recharge' these holes in summer with hot water but experience has shown that deepening the bore holes and optimizing the arrangement should serve the same purpose. This work has been carried out by the Swedish State Power Board; since 1980, 450 demonstration heat pump plants have been installed with loadings from 1.5 kW to 11 MW.

Energy technologists have for many years considered the waste cooling water from turbo-generators to be an obvious source of constant temperature input for heat pump operation and one that would be available for district heating. This seems obvious and convenient but is not all that sensible in terms of energy strategy. In the overall plan for maximizing value from fuel, it would be better to convert the condensing turbine concerned into back-pressure, sacrificing the minuscule loss of electricity involved and distributing the total heat gained without the intervention of condenser and cooling tower. Sulzer- Escher Wyss in the FRG have completed the heat chain using the longer route, however.

At Goppingen in the FRG the local gelatine producer has cooling pond water available free all year round. The gas utility, Filstall, has installed two engine-driven compressors of l l0kW, each raising the water temperature to 85°C for circulation, returning at 50°C. The maximum heat load is 3630 kW, serving an office block, school, swimming pool and gymnasium. Extensions to the system will later include business premises and apartments.

It would seem a pity to spoil an object lesson in energy planning by adding to the above but in this case at Goppingen the cooling tower water comes not from a turbine but from the gelatine evaporation process, one which cannot be modified to optimize fuel consumption in that phase of operation. As an example of energy conservation taken to its logical conclusions, this entreprise succeeded in attracting the German gas industry's prize for rational use of energy in 1984.

This review, for reasons of space, is by no means comprehensive; the volume of papers now available* covers more ground in much greater detail. The final words, in the context of achieving high COP, must be reserved for a paper that describes a simple and cheap method of increasing the effectiveness of heat exchangers using fluorinated hydrocarbons, substances having essentially high dielectric properties. By operating the heat exchanger in a highly charged atmosphere using an electric potential of several kilovolts - up to 30 kV - an increase in heat transfer of up to 10 times can be obtained. The field effect leads to nucleate boiling and, although much the same result can be obtained by mechanical agitation of the heat exchanger, it is anticipated that this purely static method should speedily achieve commercial application. This work has been carried out by Allen and Cooper of the London City University, supported by the UK Science and Engineering Research Council.

Norman Jenkins International Energy Strategy, Surrey, UK

* The proceedings volume is now available from BHRA, The Fluid Engineering Centre, Cranfield, Bedfordshire MK43 0AJ. UK