The appraisal of three gas-fired small-scale CHP systems.

Abstract

The research in this thesis has undertaken a technical. economic and environmeiital appraisal of three gas-fired, small-scale Combined Heat-and-Power (CHP) systenlýý together with a study of the UK's electricity supply industry (ESi) and CHP market. The purpose of each system is to attempt to utilise more of the heat and/or electricitY output from the CHP unit. Within the non-technical research area, t hree scenarios for the evolution of the ES1 have been developed to help establish llow changes to forces acting within the industry, might affect the development of the UK CHP market. New applications of several strategic management, alialysis tools were used to develop and select the following scenarios: (i) 'N-ewa nd reduced ('02 limits set by the Climate Control Conference + stricter environmental legislatioil, (ii) Changes to the Pool mechanism for pricing electricity. (iii) Business as usual. It was concluded that in isolation scenarios I and 3 would aid the expansion of the, CHP market, whereas scenario 2 is likely to hinder it. The selection of the scenarios and the implications for the ESi and CHP market are supported by the opinions of 'industry specialists', which were solicited in a survey specifically undertaken for this study. The investigation into the first of the three technical systems involves the substitution of two separate CHP units in place of a single larger unit. The intention is to operate the larger of the two CHP units at maximum output to satisfy the base heat-load and to use the second unit for meeting peak loads. The results for five test-cases were produced via a newlY-developed predictive model, and indicated that it is possible, for one of the case studies considered, to achieve shorter pay-back periods when using the double-unit - with a higher availability of 9.5% - rather than the single-unit system. In the other two cases (where CHP is a viable economic option), longer pay-back periods ensue by the installation of the twounit rather than the single-unit system. The operation of the two-unit system call potentially increase energy-utilisation from the CHP units at one of the other sites'. Furthermore, the proposed system can offer, in some cases, significant secondarý' benefits, which could encourage a potential investor in the technology. These benefits include the increased heat- an d-elect ri city output, increased availability from the system, back-up from the secondary unit if one unit fails. The second system determines the viability of an integrated small-scale CHP and TES system. Another predictive model was developed and tested on five test -case",. It was found that there is insufficient potential for the system and that the pot(, iitial is limited by the following factors (i) CHP-sizing methodology, (ii) the relat IvCIN, high capital cost for TEs hardware and installation, (iii) the relatively low econwilic value attributed to heat and (iv) the availability of IoN%-pricedo ff-peak electricitv. An industrial case study provided a rare and useful operational exainple of tlic proposed system and the findings indicated that the heat-store could reduce i he energy and monetary expenditures by up to 2.8/7c of the site's annual gas usage. displacing approximately 30 tones Of C02 emissions each year. Howe\-er, becauýw of the high financial cost of the TES components and installation. the pay-back period produced would rarely be acceptable to a prospecti\-e investor. except in exceptional circumstances. Finally, the viability of an integrated CHP/absorption chiller systeni was in\-(, stigated. The effectiveness of these types of systems are dependent on several factors, namely: the source-water temperature from the hot-engine CHP unit - for a high cop - and the cooling load at the site, the cooling demand at the site and the temperature of the cooling water. A first-stage predictive model was developed to determine the initial appropriateness of the installation of the integrated system at a local hospital for the first time. The indications were that the cooling demand was too low and the surplus waste-heat from the CHP unit insufficient to make the system viable at the site. A second working-system was studied with a full ('02 investigation undertaken. The intention was to compare the total C02 emissions for the integrated CHP and absorption chiller system with those for a similarl. y sized vapour-compression system. The results indicate that the installed systc1l) will produce 0.30kgCO2/kWhcoolth compared with 0.27 kg and 0.32kg for two different types of vapour compression systems at design conditions. If the CHP heat output is increased - to supply all of the heat required by the absorption chiller - then the proposed system can displace up to 0.06 kgC02 per kWhcoolth at design conditions and 0.10 kgC02 per kWh of cooling delivered for lower cooling water temperatures. This represents a reduction of 22% and 40% respectively, when compared with the vap our- compressions system.