The appraisal of three gas-fired small-scale CHP systems
The research in this thesis has undertaken a technical, economic and environmental appraisal of three gas-fired, small-scale Combined Heat-and-Power (CHP) systems 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, three scenarios for the evolution of the ESI have been developed to help establish how changes to forces acting within the industry might affect the development of the UK CHP market. New applications of several strategic management analysis tools were used to develop and select the following scenarios: (i) New and reduced CO₂ limits set by the Climate Control Conference + stricter environmental legislation; (ii) Changes to the Pool mechanism for pricing electricity; (iii) Business as usual. It was concluded that in isolation scenarios 1 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 95% - 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 two unit rather than the single-unit system. The operation of the two-unit system can potentially increase energy-utilisation from the CHP units at one of the other sites. Furthermore, the proposed system can offer, in some cases, significant secondary benefits, which could encourage a potential investor in the technology. These benefits include the increased heat-and-electricity 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-cases. It was found that there is insufficient potential for the system and that the potential is limited by the following factors: (i) CHP-sizing methodology, (ii) the relatively high capital cost for TES hardware and installation, (iii) the relatively low economic value attributed to heat and (iv) the availability of low-priced off-peak electricity. An industrial case study provided a rare and useful operational example of the proposed system and the findings indicated that the heat-store could reduce the energy and monetary expenditures by up to 2.8% of the site's annual gas usage, displacing approximately 30 tones Of CO₂ emissions each year. However, because of the high financial cost of the TES components and installation, the pay-back period produced would rarely be acceptable to a prospective investor, except in exceptional circumstances. Finally, the viability of an integrated CHP/absorption chiller system was investigated. 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 CO₂ investigation undertaken. The intention was to compare the total CO₂ emissions for the integrated CHP and absorption chiller system with those for a similarly sized vapour-compression system. The results indicate that the installed system will produce 0.30kg CO₂/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 kg CO₂ per kWhcoolth at design conditions and 0.10 kg CO₂ per kWh of cooling delivered for lower cooling water temperatures. This represents a reduction of 22% and 40% respectively, when compared with the vapour-compressions system.