Modeling and Exergy Analysis of District Cooling

Chan Nguyen

Publikation: AfhandlingPh.d.-afhandling


In this thesis energy, exergy and exergoeconomic analysis has been carried out on a different number of co-generation energy systems involving cooling. The models and methods developed can be used as a frame work to improve the district heating and cooling system thermodynamically and/or economically which is the objective of the PhD project.

A thermodynamic (energy and exergy) model of a transcritical CO2 cooling and heating system has been developed. The coefficient of performance (COP) of the system is the characteristic of interest. A sensitivity analysis of the parameters: compressor isentropic efficiency, effectiveness of the internal heat exchanger, pressure losses and the pinch temperatures in the heat exchanger on the COP has been carried out. The results show that the COP is most sensitive to the following order of parameters: compressor isentropic efficiency, pinch temperature in the gas cooler, pinch temperature in the evaporator and effectiveness of the IHX. These results are complemented by the exergy analysis, where the exergy destruction ratio of the CO2 system’s component is found.

Heat recovery from vapour compression heat pumps has been investigated. The heat is to be used in a district heating system based on combined heat and power plants (CHP). A theoretical comparison of trigeneration (cooling, heating and electricity) systems, a traditional system and a recovery system is carried out. The comparison is based on the systems overall exergy efficiency. The traditional system consists of a combined heat and power (CHP) plant with a separate refrigeration plant, where its condenser heat is rejected to the environment. The recovery system consists of the same CHP plant but with a heat pump, where the condensation heat is recovered.

Five different refrigerants (R717, R600a, R290, R22 and R143a) are chosen to be representative for current refrigeration plants of the traditional and recovery system. Also different refrigeration cycle, one and two stage cycle is considered. The CHP plants considered is back-pressure and extraction plant.

In general heat recovery is more beneficial if the district heating system is based on back-pressure rather than on extraction CHP plant. Heat recovery with extraction CHP plant is in general questionable. If heat recovery is considered it is recommendable to use two stage cycle rather than one stage cycle heat pump.

Apportioning the costs of different energy services which are produced by the same energy system is not trivial. As an example the cost of heating and cooling provided simultaneously by an ammonia heat pump has been apportioned with two costing methods – energy and exergy costing. Parametric study on the heating, cooling and surrounding temperature has been carried out. It has been demonstrated that the two methods yield significantly different results. Energy costing prices the unit cost of heating and cooling equally independent of the quality of the heat transfer, and it tends to overprice the cost of cooling in an irrational manner. Energy costing will also not encourage rational heating and cooling temperature demand and thus will not promote efficient use of resources. These flaws are not seen with exergy costing, since it has taken the quality of heat transfer in to account. Consequently the exergy costing method is found to be the more rational apportioning method for simultaneous district heating and cooling.

The methodology of the exergy costing method can also be used to calculate the environmental impact of each consumer. Taxation can eventually be based on this. The method is called exergoenvironmental analysis. As a principle example, the CO2 emission for each of the cooling and heating consumer is found. The conclusion is analogue to the exergy costing method, i.e. the exergoenvironmmental method can be used as motivation for reducing CO2 emission.

One of the main obstacles with district cooling in a traditional water based system is the investment cost for the pipes. To overcome this, a combined district heating and cooling system based on CO2 as refrigerant and transport fluid is proposed.

Exergoeconomic analysis has been used to evaluate and optimize a CO2 based system for combined heating and cooling. The exergoeconomic method SPECO is used. The system has variable demands which lead to structure change, a difficult challenge for exergoeconomic methods. Structure change in an energy system is when not all of its components are operating in the same time and/or the direction of the fluids change direction. For handling these issues time weighted average of the exergoeconomic variables has been calculated. Based on these variables iterative optimization is carried out. For comparison the CO2 system is also optimized by a direct search method, which results are considered as the true optimized value within four significant digits. The exergoeconomic method tends to converge to the results of the direct search. The computing time is lower for the exergoeconomic method compared to the direct search method. The difference will be more pronounce in favor of the exergoeconomic method, when the system’s complexity increases. It can therefore be conclude that the exergoeconomic method using time weighted average exergoeconomic variables is applicable for the CO2 system with varying demands and structure change.
StatusUdgivet - 2014