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, EstoniaAPPLICATION OF EXERGOECONOMICS TO THE OPTIMIZATION OF BUILDINGHEATING SYSTEMS CONNECTED TO DISTRICT HEATING NETWORKSC. W. Snoek and S. C. KluitersRenewables and Integrated Energy Systems, CanmetENERGY, Natural Resources Canada,1 Haanel Dr, Ottawa, K1A 1M1, CanadaABSTRACTThe concept of energy efficiency, defined as usefulenergy output as fraction of required energy input, hasbeen used for years in technical systems assessments.In addition to energy efficiency, there are benefits tousing exergy efficiency to assess system performance.Whether systems will be installed or not is ultimatelydetermined by their economic performance. Thisperformance is usually determined by comparing initialinvestment cost and operational cost with revenuesthroughout a system‘s lifetime in terms of payback timeor net present value.This paper describes a novel methodology that usesthe concept of exergy and the thermoeconomic factor,a ratio that compares investment-related cost andexergy destruction cost, for the economic optimizationof a community energy system. It compares the cost ofexergy and the required capital and operational costsincluding carbon taxes to accommodate this low qualityenergy. In doing so it enables a quick way to properlyassess the value of a system‘s ability to use low exergyenergy inputs. The method is compared to a moretraditional economic analysis.INTRODUCTIONIn the last few years, we have become painfully awareof the effects of climate change. The burning of fossilfuels and the resulting emissions are thought to be amajor contributor to the apparent increase of adverseweather events. While people need energy for comfort,in some cases there may be a choice in the source andnature of that energy. In addition to climate change,there is also a concern about the rapid depletion of themore valuable of fossil fuels, natural gas and oil. Forthese reasons it makes much sense to re-evaluate thesources of the energy we use and the effect of usingthem has on the environment.To lower energy requirements, energy efficiency hasbeen practiced for many years. In terms of comfortheating in houses, most of the effort has gone intoimproving building insulation, better windows, buildingorientation with respect to the sun, shading from solarenergy etc. In terms of energy conversion equipment,improving the efficiency often meets ‗natural‘ limits,such as those expressed by Carnot‘s Law.45Often, omitted from consideration is the ―quality‖ of theenergy that is needed to provide comfort to theoccupants of a building. While the heatingrequirements of a building can be determined (in GJ orTJ), the nature or origin of this energy is not addressedin energy efficiency calculations. The total amount ofJoules can be provided by oil, natural gas, electricity orlow temperature ‗waste‘ heat. While the first threeenergy sources are considered high quality, and can beused to generate very high temperatures (over1000 °C), run equipment such as computers, radio andTV transmitters and receivers, ‗waste heat‘ is of lowquality and has no other use. Comfort heating does notrequire high temperatures and therefore using highquality fuel for low quality applications is consideredwasteful.Energy quality is often expressed as ‗exergy‘. Exergy isdefined as the maximum useful work possible during aprocess that brings the system into equilibrium with aheat reservoir. To illustrate the concept of exergy onecan compare two different forms of the same amount ofenergy: 100 kJ of energy is equivalent to:– 12 V/2.3 Ah stored in a car battery, or– 1 kg of water at 43 °C in a room with an ambienttemperature of 20 °C.Obviously, the energy contained in the battery isconsidered more useful and therefore has the higherquality or exergy.The ratio of Exergy (E) to Energy (Q) can be expressedas:EQTambient 1(1)Tsup plywhere T is given in K.Equation 1 shows that when the supply temperature ofan energy source is high, the exergy converges to theenergy value. Electricity and mechanical work are(nearly) perfectly convertible and the exergy content istherefore equal to the energy content. Conversely,when the supply temperature is closer to theenvironmental temperature, the value of the exergybecomes (much) smaller than that of the energy.Wall [1], in his paper on ―Exergy and Morals‖ quotesAlfven who claimed that energy accounting based onenergy only is like a bank teller counting by the amount