Thermodynamic evaluation of utilizing different ice thermal energy storage systems for cooling application in office buildings in Malaysia

Abstract

Storage can establish balance between production and demand consumption level in almost all the energy conversion systems. The same principle is valid for cooling applications, especially when the system is supposed to operate daily during the year. This is the condition that exists in tropical climate of Malaysia. The statistical data shows that almost one-fourth of the AC energy use in the country is due to office buildings. Therefore, utilizing the cold thermal energy storage (CTES) technique can significantly reduce the energy demand. In this study, a macroscopic thermodynamic analysis of the application of five different CTES systems for an office building in Malaysia is presented. The building energy usage is recorded and the average pattern is applied for chiller selection, storage tank sizing and finally energy and exergy evaluation. The results show that all the systems are highly efficient in terms of energy with the minimum of 93% for ice harvesting and maximum of 98% for encapsulated technique. However, the exergetic evaluation implies a totally different scenario of the study. The maximum exergy efficiency is for ice on coil (internal) technique with an amount of 18%. It was also found that increasing the room set-point temperature by 5 °C can reduce the exergy efficiency by 4%.

Introduction

Malaysia, a country located near the Equator with tropical climate has an average temperature fluctuation of 20–32 °C and an average rainfall of 3540 mm per annum [1]. Like every other developing country, Malaysia has experienced rapid economic growth in the past decade and consequently its national electricity usage in the residential sector has been boosted up remarkably. Due to the tropical climate within the region, one of the main electrical appliances operating in the residential and commercial sector is AC systems with 16–50% share [2]. In Malaysia, AC systems are considered as the major energy users in office buildings with almost 57% share on electricity bill [3]. The total number of AC systems augmented from around 13,000 units in 1970 to more than 250,000 units in 1991 and the number is predicted to exceed over 1.5 million units by the year 2020 [4]. Consequently, the reported electricity usage of the AC systems increased from 1200 GWh in 1999 to more than 2200 GWh in 2009 and it is predicted to exceed more than 3000 GWh in 2015 [5].

AC systems normally operate during a few hours of the day with their full load. CTES is a technology whereby cold energy is stored in a thermal reservoir during off-peak periods for later use [6], [7]. Consequently, shifting electrical demand to off-peak hours has a major “Green” benefit of decreasing fuel consumption in the power plant [8], [9]. Moreover, electricity production and distribution during the night hours, when the ambient temperature and line losses are lowest, is significantly more efficient than those for daytime. Many electricity providers have recognized the potential of CTES systems to change electricity expenditure patterns, and now offer special pricing structures as incentives for energy users to deploy these systems. Based on the latest information released by TNB Malaysia, the main Malaysian electricity provider [10], the peak period for medium voltage commercial use (C2) is from 8:00 to 22:00 (14 peak hours per day). However, to encourage customers to shift their electricity usage to off-peak hours, TNB offers special rate structures for those using thermal energy storage systems. Based on the special rates, the peak period is reduced by 2 h (from 9:00 to 21:00). The electricity tariff rate and the maximum demand charge for the conventional AC system are $0.104 (RM0.312) and $8.633 (RM25.90), respectively compared to $0.06 (RM0.182) and $12.86 (RM38.60) for the case of CTES systems ($1 is equal to RM3) [11], [12].

The importance of demand reduction and stability of the electric grid has made the energy storage technology a valuable technique capable of balancing the electricity production and demand. Nowadays, CTES systems are widely used in different applications for the buildings that are mainly occupied during the working hours, such as office buildings [13], [14], hospitals [15], schools [16], [17], churches and mosques [18]. The storage medium can be ice, chilled water or eutectic salt PCM [19]. Based on the storage medium, CTES systems are categorized into three major types of ITS, CWS and phase change material thermal energy storage systems [20]. For projects where space is not a major issue, CWS systems are commonly used. However, in retrofit projects where space is limited ITS systems are considered as the best solution [8]. The operating strategy of CTES systems are generally divided into two categories of full and partial storage, Fig. 1.

Several investigations and case studies about CTES technology and related strategies are available in the literature [21]. A powerful method to evaluate the performance of the CTES system is the energy and exergy analysis on the basis of thermodynamics laws.

Exergy is associated to the concept of quality of energy. Hence, exergy evaluation has been widely used to find the most rational use of energy [22]. Exergy analysis is used to determine the exergy destruction sources and to improve the exergetic efficiency of the system. The mathematical formulation and performance analysis of a two-stage thermal energy storage unit during the charging and discharging stages were performed by Domanski and Fellah [23]. Applying the definition of entropy generation which was presented by Benjan [24], the exergy efficiency was calculated. Investigating the effect of mass velocity on the exergy efficiency, it was found that increasing the coolant flow rate would decrease the exergy efficiency. It was also pointed out that the highest exergy efficiency would be obtained when the melting temperature of the downstream unit was close to the ambient temperature. The performances of four different CTESs based on the first and second laws of thermodynamics was assessed by MacPhee and Dincer [25]. The reported total energy efficiency for full storage was a little higher than the partial storage. The maximum energy efficiency was reported to be 99.02% for ice slurry storage system followed by ice on coil (internal melt) and encapsulated ice storage system with 98.92%. Exergy efficiency was defined as the ratio of the desired exergy to the required exergy. The variable of the presented case studies are similar to the study conducted by Dorgan and Elleson [20] and Wang and Kusumoto [26]. The total exergy efficiency of ice on coil (internal melt) was stated to be the maximum and was equal to 14.05% during full storage load and 13.9% during partial storage load.

According to statistical data, about 21% of the total electricity consumption in Malaysia belongs to office buildings [5]. Therefore, there is a great potential to reduce utility costs as well as energy expenditure and carbon emissions in this sector. The main objective of the present work is to investigate the energy and exergy efficiencies of five commonly used ITS systems namely; ice on coil-internal, ice on coil-external, ice slurry, encapsulated ice and ice harvesting, for a case study office building in Malaysia. To the best of authors’ knowledge there is no experimental work available on energy and exergy evaluation of CTES systems based on Malaysia climate. Therefore, this work is expected to fill this gap. Furthermore, the effects of ambient temperature fluctuations and room set-point temperature variations on energy and exergy efficiencies are investigated.