Comparison of energy performance and economics of chilled water thermal storage and conventional air-conditioning systems

During the summer of previous years, Kuwait faced a series of power shortages emphasizing the need for urgent commissioning of power generation projects. It is estimated that the demand for electricity is growing at an average of 5.4% per year, encouraged by government subsidies and driven by the rapid and continual expansion in building construction, urban development, and heavy reliance on air-conditioning (AC) systems for the cooling of buildings. The chilled water thermal storage (CWTS) system is one of the available techniques that can be utilized to reduce peak electricity demand of buildings when national electricity consumption is at its highest level. This paper demonstrates that the implementation of CWTS system reduces the peak power demand of AC systems for design day conditions by 36.7–87.5% and annual energy consumption by between 4.5% and 6.9% compared with conventional systems, where chillers and pumps significantly contribute to this reduction. In addition, the Life Cycle Cost (LCC) was estimated for both the Ministry of Electricity and Water (MEW) and the consumer. Results show that CWTS operating with a load leveling strategy gives the lowest LCC compared to 50% demand limiting and full storage strategies, and is, therefore, considered as the most cost effective option for both MEW and consumer.

MEW, the sole supplier of electricity in Kuwait, is facing huge increase in demand year on year. This increase in power demand is due to rapid growth in constructions of residential buildings, modern offices and large commercial buildings which lead to high energy consumption for the operation of AC systems. Based on statistical data supplied by MEW [1], the peak power demand has significantly increased from 6.45 GW in 2000 to 10.90 GW in 2010 with annual average increase of 5.3% which is well above the world average of 2.7% [2]. Peak power demand is important because MEW is faced with investing in new generation capacity to cope with expected increase in peak demand. In addition, it has been estimated that AC systems of buildings annually consume about 45% of the exported electrical energy from the power plants and contribute about 63% of the peak power generation [3]. Cool thermal storage system is one of the well known technology that can be implemented to significantly reduce the electric peak power demand of the AC systems in buildings. The primary benefit of implementing of this technology is to shift the power consumption of the AC systems from the daytime to nights when both the demand of electricity and temperature are considerably lower [4]. In addition, some systems configurations and designs may result in lower capital and operating costs [5]. Depending on the system configuration, the chiller may be smaller than would be required for direct cooling, leading to smaller auxiliaries such as the cooling tower, the condenser and the chilled water pumps [6]. Pumping energy can be reduced by increasing the chilled water temperature range, and fan energy can be cut with colder air distribution as the case when ice thermal storage is used [7]. Al-Rabghi in Ref. [8] argued that thermal energy storage could be one of several technological methods for lowering energy consumption of buildings if it were incorporated within their AC systems. Moreover, Henze et al. [9] showed that the adoption of cool thermal storage system within chilled water plant for a group of buildings in Germany provide economic benefits, operational merits by avoiding numerous safety measures necessary for a cooling plant without storage and a cost effective addition of supplemented cooling capacity. In another application, Ehyaei et al. [10] studied the effect of utilisation of cool thermal storage system on the selection of micro gas turbine for electricity generation for a residential building located in Tehran. Their study showed that cool thermal storage reduces the micro gas turbine units from 21 to 11 and cost by 29.5% due to reduction in maximum cooling demand of the building. In addition, further case studies of implementing cool thermal storage have been presented by Yau and Rismanchi [4] as well as successful implementation for other applications given by Dincer and Rosen [11]. Cool thermal storage system is accepted by many as a proven energy conservation technology since it reduces the energy consumption and hence results in conservation of fossil fuels and reductions in green house gases and CFC emissions [12]. The advantages of a cool thermal storage system over a conventional AC system are summarised below [13]: • The refrigeration capacity can be substantially reduced. • The chiller plant operates at its optimum efficiency. • The chiller efficiency can be improved and a constant generating load can be maintained. However, there are some practical difficulties in implementing the technology. These difficulties arise from errors in the sizing of the system which result in higher payback period, failure in the operation of the system due to mechanical malfunction of equipment and control system and an inexperienced operator causing inefficient operation of the system [13]. In Kuwait, there is no cheap rate electricity tariff and there is no direct cash incentive offered by MEW for demand management measures. However, the cool thermal storage system may be attractive for both MEW and consumer if peak power demand and energy consumption are both reduced. Many types of cool thermal storage technologies are available in the market; however, CWTS is the most promising storage technology due to its lower initial cost and electrical energy when compared to other ice storage technologies [14]. In addition, CWTS does not need special equipment, can be incorporated with existing conventional AC systems and reliable with good track record [15]. Therefore, in this paper the energy performance and economics of using CWTS assisted AC system against conventional AC system are studied for Centre for Speech and Audio Therapy (CSAT) building in the climate conditions of Kuwait. The CSAT building which represents typical medium sized building (see Fig. 1) was selected to assess the impact of cool thermal storage on the electrical power and energy consumption of the air cooled chillers. The building comprising of two blocks, referred to as A and B, connected by a small corridor. Block A is a single story construction located at the rear part of the building. Block B has ground and first floors in addition to a tall reception with a large glassed area including a skylight. This building is occupied from 07:00 a.m. to 02:00 p.m. for five days a week, and has a total floor area of 3180 m2.

In Kuwait, the demand for electricity has recently become a topic of major interest and prominence. Electricity is a highly capital-intensive sector, which requires relatively long lead times for construction and development. The gap between supply and demand is shrinking and new projects require huge investment, the financing of which faces a dilemma in the light of high fluctuations in oil revenues. In this paper, the building energy simulation program, ESP-r, was used to determine the hourly cooling demand of a typical medium size clinic building for the design day condition and for the whole year using the typical metrological weather data for Kuwait. The simulations were conducted with and without ventilation control. The effect of the ventilation control showed a reduction in the range 5.7–4.3 MWc h in the cooling demand during the unoccupied period. This made the use of cool thermal storage more attractive because of the lower load during the time at which the storage is charged. Further reduction could be achieved in the charging period during night time, if high thermal performance windows with low overall heat transfer coefficient were applied. In addition, each component including chillers, AHUs and pumps in the AC systems were sized; the heat gains by these auxiliary systems were then determined and added to the building cooling demand. Thus, the system cooling load of the building was developed for all AC systems. The input power requirements of the components were determined for the design day condition and for the whole year using bin-bases analysis using method. For the design day conditions, it has been found that the peak power demand CWTS AC systems chillers was decreased by between 36.7% and 87.5% depending on the design of the operating strategies. Full strategy was found to have the largest reduction in the peak power demand of 87.5% compared with conventional AC system. This reduction was established as a result of switching off the chiller and primary pump completely during the day time. Furthermore, the results for the annual performance of the CWTS AC systems were found to be better than the conventional system. The annual energy consumption was improved by between 4.5% and 6.9% depending on the operating strategy. Lower chillers and primary pumps sizes and slight improve in chiller's COP has contributed to this reduction. Economic analyses were also carried out for each AC system to assess the LCC based on the actual estimated costs by the MEW for the water, energy consumptions and power connection and on the subsidized costs to the user. For MEW, the LCC for all CWTS AC systems were found to be lower by between 4% and 8.9% compared to the conventional AC system. This was mainly contributed by the reduction in capital and energy costs. In addition, estimated LCC for MEW and consumer have shown that the CWTS operating with a load leveling strategy is the most cost-effective design for the clinic building when air-cooled chiller is used. Based on the above findings, the CWTS AC system operating with load leveling strategy represents the most cost-effective method for both the MEW and user for the building type that was selected for the study.