12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, Estoniaof coins and bills, but neglecting their value. In anethical society the value, worth and quality of differentenergy supplies should, as a minimum, be matched tothe requirements of the different energy applications.Methods to design low exergy buildings are availabletoday. For instance, Schmidt [2] developed a methodand pre-design tool for low exergy buildings in which hecompared different heating systems, such as boilers,condensing boilers, electric heating, GSHP and lowtemperature under-floor heating. However, this methoddoes not directly address the effect of system heattransfer surface area on the overall economics.Also, there is an additional benefit realizing that abuilding that can accommodate low exergy streams isready for future hook-up to other, perhaps renewableenergy sources: GSHP, solar, waste heat fromindustry, energy from thermal storage to name a few.This is a distinct advantage when the move to asustainable society gains momentum, and the conceptof low-temperature heating should be incorporated inbuilding codes.This paper considers the cost of using the low qualitypart of the energy source and the (increased) capitalcost and operating cost that are required to‗accommodate‘ low quality energy. A methodology hasbeen developed to determine the optimal cost ofoperation, based on the capital cost, operational costand the cost of the exergy.This type of analysis is considered part of the field ofthermoeconomics, more in particular exergoeconomics.Wikipedia defines thermoeconomics in a verytheoretical way as a school of economics that applieslaws of thermodynamics to economy. Valero et al. [3]operationalize this definition by describing two aims ofthermoeconomics, (1) optimization to minimize cost ofa system or component, and (2) cost allocation ofindividual outputs of a plant producing a number ofoutputs.Valero and coworkers [3] date this research field backas far as 1932, when Keenan apportioned cost of heatand work taking into account irreversibility andthermodynamic efficiency instead of enthalpy only.However, they go on to say that Gaggioli, and Tribusand Evans in the early 1960s started off realdevelopment in thermoeconomics. Ever since, thesefields have received tremendous attention. Valero andcoworkers identify that an important problem in thisbody of research is the variety of methodologies usedwith accompanying nomenclature. Between them andTsatsaronis [4] they already name a fair amount ofmethods. In doing so, Tsatsaronis introduces theexergoeconomic factor f, as a fraction that comparestwo sources contributing to cost increases, investmentrelatedcost and exergy destruction cost. Thisexergoeconomic factor is also found in other sources,such as Temir & Bilge [5].It is beyond the scope of this paper to provide acomprehensive literature overview of thermoeconomicpublications or even of the methods used in thesepublications. The aim of this paper is to apply one ofthese methods, using the above-mentionedexergoeconomic factor to optimize building heatingsystems connected to a district heating system. To thebest of the authors‘ knowledge, so far this method hasonly been applied to optimize individual components.This work ties in with research into advanced lowtemperaturedistrict energy systems currently carriedout at the CanmetENERGY laboratories of NaturalResources Canada in Ottawa, Canada.The system considered consists of buildings with theirheating system (radiators and cross-flow heatexchangers are considered), the energy centre withboilers and pumps and the pipeline to move the energyin the form of hot water to the community. Thedevelopment of the methodology was the main objectof the study, not the optimization itself.While the development of the optimization was relatedto economics, in other words, the least costly option, itshould be noted that the concept of ‗exergy‘ opens upthe notion of ―morals‖ and ―ethics‖. For newdevelopments, the costs of resource depletion andenvironmental destruction should be considered aswell. Just because a certain system is economic, it isnot necessarily the best moral or ethical choice. Justbecause a certain system does not cause localproblems, that does not mean that (environmental orother) problems caused by this system elsewhere canbe ignored.Traditional OptimizationsSystem optimization is often done by optimizingsystems separately, and not by considering the overallefficiency of integrated systems. Often, an integratedapproach leads to optimal solutions, as in electricitygeneration using a back pressure steam turbine.Accepting a lower efficiency of the turbine may lead tothe residual energy in the condenser being useful inother applications, whereas in the separately optimizedversion this thermal energy would be useless. In thelatter case, the turbine back pressure is kept as low aspossible, to extract the maximum electrical power. Thismakes the condensate of too low a temperature to beuseful in other applications. Optimizing integratedsystems as a whole avoids this problem.Exergoeconomic OptimizationIn an exergoeconomic optimization, the concept ofexergy is used to determine the best and mosteconomic solution to an energy conversion process or46