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

摘要

The research in this thesis has undertaken a technical. economic and environmeiitalappraisal of three gas-fired, small-scale Combined Heat-and-Power (CHP) systenlýýtogether 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 electricitYoutput from the CHP unit. Within the non-technical research area, t hreescenarios for the evolution of the ES1 have been developed to help establish llowchanges to forces acting within the industry, might affect the development of theUK CHP market. New applications of several strategic management, alialysis toolswere used to develop and select the following scenarios: (i) 'N-ewa nd reduced ('02limits 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 scenariosand the implications for the ESi and CHP market are supported by the opinions of'industry specialists', which were solicited in a survey specifically undertaken forthis study.The investigation into the first of the three technical systems involves the substitutionof two separate CHP units in place of a single larger unit. The intentionis to operate the larger of the two CHP units at maximum output to satisfy thebase heat-load and to use the second unit for meeting peak loads. The results forfive test-cases were produced via a newlY-developed predictive model, and indicatedthat it is possible, for one of the case studies considered, to achieve shorterpay-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 viableeconomic option), longer pay-back periods ensue by the installation of the twounitrather than the single-unit system. The operation of the two-unit system callpotentially 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 benefitsinclude the increased heat- an d-elect ri city output, increased availability fromthe system, back-up from the secondary unit if one unit fails.The second system determines the viability of an integrated small-scale CHP andTES 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(, iitialis 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 econwilicvalue 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 tlicproposed system and the findings indicated that the heat-store could reduce i heenergy 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, becauýwof the high financial cost of the TES components and installation. the pay-backperiod produced would rarely be acceptable to a prospecti-e investor. except inexceptional 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 highcop - and the cooling load at the site, the cooling demand at the site and thetemperature of the cooling water. A first-stage predictive model was developed todetermine the initial appropriateness of the installation of the integrated systemat a local hospital for the first time. The indications were that the cooling demandwas too low and the surplus waste-heat from the CHP unit insufficient to make thesystem viable at the site. A second working-system was studied with a full ('02investigation undertaken. The intention was to compare the total C02 emissionsfor the integrated CHP and absorption chiller system with those for a similarl. ysized 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 differenttypes of vapour compression systems at design conditions. If the CHP heatoutput 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 designconditions and 0.10 kgC02 per kWh of cooling delivered for lower cooling watertemperatures. This represents a reduction of 22% and 40% respectively, whencompared with the vap our- compressions system.