Aprovechamiento del calor residual a baja temperatura mediante bombas de calor para la producción de agua caliente.

Abstract

A significant percentage of energy is destined to produce Domestic Hot Water (DHW) within the building sector. Furthermore, most of that energy contained in the water is wasted to the ambient after its use. Heat pumps have been clearly identified as an efficient technology for DHW production, and as a main vector towards future de-carbonization of cities. In addition, they could use the heat from the wastewater as a heat source. Thus, contributing in two ways towards a more environmentally friendly energetic sector. However, the use of heat pumps for DHW recovering heat from wastewater faces several challenges that require further analysis and development: 1. A heat pump design capable to operate with high performance when variable secondary temperature lifts at the heat sink take place. 2. A heat pump design capable to operate with high performance when variable secondary temperature lifts at the heat source take place. 3. The integration of the heat pump within a system (heat recovery strategies, components, sizing, operation strategy). Usually, transcritical cycles have been considered the most suitable cycle for DHW production (high temperature lifts of the heat sink, 10-60°C). However, this cycle involves several drawbacks as for instance the requirement of high pressures in the installation or a significant reduction of the performance with the increase of water inlet temperature at the condenser. Instead, subcritical cycles have demonstrated great potential for DHW applications if a proper control of subcooling is performed. The objective of this thesis is to investigate the most efficient water-to-water heat pump working with a subcritical cycle for DHW production using as a heat source wasted heat at medium-low temperature and to determine the most efficient system based on heat pumps for this application. The work is divided in two differentiated parts: ? Heat pump concept This development is a continuation from the PhD work of M. Pitarch [1]. In that PhD work, the role of the subcooling in the performance of a subcritical heat pump for DHW applications was investigated. Two different configurations of a heat pump prototype were designed based on the way subcooling was made. The results showed that a subcooling optimized subcritical heat pump was able to provide comparable performance than present HPs employing transcritical cycles. However, both configurations require one more component than usual heat pumps. Thus, a new prototype based only on the typical components (compressor, condenser, expansion valve and evaporator) was proposed as future work. In this thesis, a theoretic analysis of the heat pump was done. A subcooling control methodology was developed and tested. The proposed prototype in [1] has been built and characterized. From all the results, the most convenient heat pump design was obtained. ? Integral Heat pump-DHW system The integration of the most convenient heat pump prototype within a system for the DHW production based on heat recovery from wastewater has been analyzed. The research has included the development of a model of the entire system in Trnsys and the optimization of the main components of the system: their sizing and their operation with the objective of reaching the maximum global efficiency of the complete system. Due to the complexity of the problem, the analysis was performed in three main steps: first, a study of the direct heat exchange,second, an study focusing on the condenser side, that is, the consideration of an infinite heat source (large availability of sewage water for instance) and third, the focus was done on the evaporator side. That is, the optimization of the complete system in which a finite heat source is considered (grey waters collected from the building for instance). The simplest and most efficient system required in DHW production and heat recuperation from wastewater has been determined.