Abstract

Issues stemming from district heating utilization during summer periods and the conversion of low-temperature heat into cold in adsorption chillers have been investigated in this paper. Due to the high vulnerability of adsorption chillers to ambient conditions, in the case of relatively low ambient temperatures, adsorption-based air-conditioning systems would be characterized by excessive cooling power. Moreover, adsorption chillers are also characterized by high investment costs and big time constants, and the vulnerabilities found in their regulatory processes have yet to be sufficiently investigated. The authors recommend the application of hybrid air-conditioning systems, consisting of adsorption and compressor chillers. The adsorption chiller works as a base while the compressor chiller contributes missing cooling power, working as a regulation unit. Sixteen configurations of the hybrid air-conditioning system have been analysed. It has been shown that 100 kW cooling power hybrid air-conditioning system, with respect to its configuration, enables the utilization of 0.5 to 0.9 TJ of low-temperature heat per year, while simultaneously providing comfortable air-conditioning. The authors have concluded that the adsorption share in the analysed hybrid system should not exceed 50%.

Introduction

In European countries characterized by moderate climate, a recommended electrical and thermal energy production technology is cogeneration. It results from high heat requirement in wintertime and stable consumption of some amount of heat in summertime. A typical profile of cogenerative heat consumption byWrocław – a middle-sized Polish city (about 600 thousand inhabitants) is depicted in Fig. 1. Similar district heating demand profile is also reported by other researchers in Netherlands (Geus et al., 2015). A ratio of winter to summer heat consumption is approximately 10:1. A significant problem of cogeneration (CHP) plants during periods of prevailing external high temperatures is found in the utilization of low-temperature heat, which has been derived from cogeneration. This heat must be delivered to the receivers as they require warm utility water. However, the minimum amount of power required for CHP plant operation has significantly exceeded receipts, particularly during the summer months.This indicates the necessity of controlled excess heat dissipation at temperatures of 60 °C and higher,what aggravates cogeneration economics. Simultaneously, the demand for air-conditioning increases during summer periods. An increased demand for air-conditioning corresponds to the additional demand of electric energy.However, cogenerative productionmust be limited due to the lack of heat demand.The perfect solution would be the implementation of technology that enables the conversion of district heating into a cold directly in the receiving end, suggested also by Daßler and Mittelbach (2012). Sorption chillers are devices that enable the conversion of heat into a cooling effect. The essence of operation of sorption chillers is based on utilizing the sorption bed capacity’s dependence on pressure and temperature.These chillers are further divided into widely spread absorption chillers and adsorption chillers. The working medium for absorption chillers is water absorbed under low pressure in water solution LiBr and which, for sorbent regeneration purposes, also desorbs when a bed is heated to a minimum of 85 °C. These chillers can be installed close to cogeneration sources, but cannot be powered by district heating, which has a temperature of approximately 65 °C during the summer (Chorowski and Pyrka, 2015). Adsorption chillers can use water as a refrigerant adsorbed under low pressure by a bed filled with silica-gel. Silica-gel regeneration is possible in temperatures as low as 50 °C (Chorowski and Pyrka, 2015; Rahman et al., 2013; Saha et al., 2001) and district heating can be used for this purpose. Principles of operation and current advances on adsorption chillers are widely discussed in Li et al. (2014); therefore, they are not presented in this paper.