Combined heat-and-power market-penetration in the UK: Problems and opportunities

Combined heat (and) power (CHP) technology is capable of providing efficient, environmentally-friendly, electricity and heat supplies. Despite being thermodynamically attractive, in the UK CHP has languished in the past and so failed to penetrate the market rapidly because of a lack of leadership by successive Governments. A frustrating chronicle for CHP as well as community (i.e. district) heating (DH) in the UK is presented. However, recent developments and a possible future scenario concerning energy supplies in the UK will change matters. It is predicted that the contribution of CHP will grow significantly as the UK energy supply business becomes increasingly customer-driven.

The total energy approach: Evolution of combined heat and power for district heating and/or cooling

Combined heat and power (CHP) generation is not a new concept, but it provides an elegant solution to some of our present fuel problems, offering, as it does, 80% or greater efficiency. However, Great Britain lags behind other European countries with respect to the rate of introduction of CHP together with district heating and/or cooling (DHC) systems. Reviews of (i) the historical development of the designs of DHC pipelines, from insulated pipes in air-filled ducts to the modern preinsulated pipes buried in the ground, and (ii) sources of energy as well as developments in metering and control, for CHP-DHC systems, are presented. The cost effectiveness of each CHP-DHC system is highly sensitive to unit fuel prices, current discount rate, as well as the capital cost incurred. In the best interests of Britain, major governmental investments are now needed urgently in order to encourage the wider adoption of these systems.

Combined heat and power as a contributory means of maintaining a green environment

Small-scale combined heat-and-power (CHP) systems are cheaper and environmentally cleaner than conventional generators and deserve to be supported by the UK electricity industry, even when privatised. A feasibility assessment for the rate of CO2 emissions from a mini-packaged CHP unit, serving a high-rise block of 126 flats, is presented. Predictions were compared with those when [`]dirty' generated electricity was consumed; the heating demand for the building being met via a central boiler or individual boilers or electric heaters.

Pre-insulated district-heating pipelines: Design and operational advice

Steady-state temperature distributions have been measured within a cylindrical duct enclosing two different diameter pipes--one for district heating and the other for the supply of domestic hot-water; the gaps between the pipes being filled with well-packed fibrous thermal insulant. The procedure was repeated successively with each of three sets of different-diameter pipes. The recommended configuration was with the DH pipe coaxial with the centre of the pipeline and the DHW one located vertically below it: this corresponded to the minimum rate of heat loss from the district-heating pipe under the considered conditions. An overall improvement of ~8% in the thermal insulation of the DH pipe has been achieved compared with the current conventional practice of installing the DHW pipe above the DH pipe.

Forced steady-state convections from pin-fin arrays

The steady-state thermal and air-flow resistance performances of horizontally-based pin-fin assemblies have been investigated experimentally. The effects of varying the geometrical configurations of the pin-fins and the air-flow rates have been studied. The optimal pin-fin separation Sxopt in the span-wise direction, to achieve a maximum rate of heat transfer from the assembly, is 1·0 ± 0·2 mm

Improved configurations for district-cooling pipelines

Visualisation of the air flows in the gaps between horizontal cylindrical pipes surrounding relatively cooler pipes has been undertaken, and the corresponding steady-state heat losses measured. Consequently, preliminary recommendations concerning how to improve the thermal insulation performances of district-cooling pipeline assemblies are presented.

District-cooling distribution network: Optimal configuration of a double-pipe system in a rectangular trench

The optimal configuration, i.e. that which achieves the minimum steady-state rate of heat gain by the (colder) supply pipe for the considered conditions, has been determined experimentally. By using the displacement ratios of the two pipes as the experimental variables, the optimal configuration of the (colder) supply and (warmer) return pipes was deduced to occur for displacement ratios of 0·67 and -0·08, respectively, i.e. with the supply pipe in the lower region of the atmospheric-pressure air-filled relatively hot trench and the return pipe vertically above it, both symmetrically placed with respect to the side walls. This optimal configuration differs significantly from the traditional side-by-side arrangement of district-cooling pipelines and should prompt designers to become more aware of the prospective energy and financial savings which be achieved.

Geometric view factors for radiation exchanges between an infinitely long cylinder and a parallel infinitely long enclosure of either cylindrical or rectangular cross-section

The view (or shape) factor for an infinitely long cylinder, enclosed in either a parallel cylindrical or rectangular enclosure, is independent of the relative disposition of the inner cylinder with respect to the enclosure or the dimensions of the system. Its value is unity.

Natural convection across cavities: Design advice

Experimental measurements and theoretical predictions of steady-state heat transfers to or from horizontal single or double pipelines enclosed in horizontal circular or rectangular enclosures have been collated. The optimal configurations of the pipelines to achieve maximum thermal resistances of the air-filled cavities are identified. A recommended correlation for predicting the combined convective/conductive resistances provided by the contained air in a horizontal concentric annuli is presented.