Evaluation of the Techno-Economic Benefits of Shifting Air Conditioning Loads from Evening and Day Peaks to Off Peak Hours A Case Study

: With the development of industries and changes in living standards of the society, demand for electricity is rapidly increasing. In order to maintain the demand supply balance and to provide uninterrupted supply, utilities have to meet electricity demand in the most economical way. Building new power plants is not always the most economical solution. The trend now, is towards reducing and controlling the demand through Demand Side Management (DSM) techniques which lead to almost always economical and environment-friendly solutions. In this paper, Heating, Ventilation and Air Conditioning (HVAC) system of Cinnamon Lakeside Hotel is analysed to identify the potential DSM options that can be implemented. Thermal Energy Storage (TES) was selected as the DSM option to store cooling load during off-peak hours of the day where electricity is fairly cheap and use it during peak and day hours when the electricity is expensive. Technical viability and potential saving that can be achieved through TES in hotel sector of Sri Lanka is further analysed .


Introduction
Thermal comfort is a main factor in modern day building designs. Air conditioners are generally used to provide thermal comfort inside buildings. Use of air conditioning is increasing around the world, contributing to a substantial increase of demand for electricity.
HVAC systems in industrial, commercial and residential buildings are the largest single contributor to electricity demand, especially during daytime [2]. Sri Lanka's daily electricity demand curve shows a predominant evening peak which is served by expensive gas turbines operated on diesel oil [5]. Therefore, all efforts must be taken to shift loads from the peak demand period and flatten the demand curve for better utilization of the capital investments.
During recent years, considerable research has been done aimed at the development of technologies that can offer reduction in energy consumption, peak electrical demand and energy costs, without affecting the level of thermal comfort. In this context, cold Thermal Energy Storage (TES) systems can play an important role as they provide great potential for improved energy efficiency, conservation and reduction in peak electrical load.
The demand for electricity in Sri Lanka is continuously increasing, growing at an average rate of 5.0% per year (20 Year growth average) [5]. Additional power plants must be constructed to satisfy this demand. It is estimated that Sri Lanka must invest around US$ 8.58 billion over the next twenty years [2] to finance an expansion program for an additional 8,363MW of generating capacity to cope with this predicted demand. Therefore, shifting air conditioning load to off peak hours using storage methods will reduce the investment on costly generation plants.
This paper presents techno-economic benefits of shifting air conditioning load from peak to off peak hours by using cold TES through a case study for a large-scale hotel.
Methodology:  Identification of the HVAC load pattern and energy consumption of the HVAC system.
 Identification of the optimum thermal energy storage required and cost-effective technology to be used for the storage.
 Evaluation of the economic benefits to the industry and to the utility.

Identification of the HVAC Load Pattern and Energy Consumption of the HVAC System.
HVAC system consumes around 60% of its total electricity consumption in a large hotel [5]. Further analysis shows that 30% to 40% of electricity consumption of the HVAC system is from the central plant which consists of chillers and cooling towers. Break down of electrical energy use in a typical hotel building is given in Figure 1 [4].

Figure 1 -Breakdown of Electricity Consumption of a Large Hotel
Existing HVAC system of Cinnamon Lakeside hotel consists of one 455 TR Single Screw DAIKIN Chiller and two 300 TR cooling towers and several pumps and fan coil units.
Two 300 TR cooling towers are operating at their full capacity throughout the day and each cooling tower is rated 7.5 kW at full load. Figure 2 shows the average load profile of the chiller at Cinnamon Lakeside Hotel. The average load profile of the chiller shows that there is a possibility of shifting cooling load to off peak hours as there is a low demand in the off-peak hours compared to the peak and day hours. The utility defines "Day" period as from 05.30 hours to 18.30 hours, "Peak" period as from 18.30 hours to 22.30 hours and "Off-Peak" period as from 22.30 hours to 05.30 hours for Time of Use (TOU) tariff.

Data Analysis
Breakdown of the average energy consumption per day of chiller and cooling towers in TOU time slots are shown in Figure 3.  CWS is a TES using sensible heat of water to store energy during off-peak hours. Vertical cylinder tanks are the most common shape of tanks used for CWS [4] and they can be located above ground, partially buried or completely buried depending on the location. Tank capacity depends on the amount of cooling load to be stored and temperature difference between stored chilled water and return water. Existing chillers can be used in this method.
Ice storage is a proven technology that reduces chiller size and shifts compressor load, condenser fan and pump loads from peak periods to off-peak periods, where electrical energy is less expensive.
The latent heat of fusion of water (phase change of water to ice or ice to water) is used in this process to store cooling load. Water is used as a phase change storage medium in order to take advantage of its higher storage capacity. In this method it is required to use glycol chillers and heat exchangers.
Following parameters are taken into account when calculating the energy consumption of each method.
Heat exchanger energy loss 01% [3] Ice Storage energy loss 01% [3] Chilled Water Storage energy loss 10% [1] 3. Identification of the optimum thermal energy storage required 3.1.1. Case 01: Shifting both peak and day cooling loads Only Glycol chillers are used and total peak and day cooling load of 6,403.7 TRh should be stored during off-peak hours.  A Glycol chiller of 1,176.80 TR (936.6 kWe) is required for the proposed case and total electrical energy consumption of the chiller is 6,556.5 kWhe.

Case 02: Shifting peak cooling load only
Only Glycol chillers are used and peak cooling load of 1,322.0 TRh should be stored during off-peak hours.

Figure 7 -Load Profile of Chiller Shifting Only Peak using Ice Storage
A Glycol chiller of 436.26 TR (347.2 kWe) is required for the proposed case and total electrical energy consumption of the chiller is 6,515.6 kWhe. Only conventional chillers are used for shifting both the peak and day cooling loads to offpeak hours using chilled water storage system.

Figure 9 -Load Profile of Chiller Shifting Peak and Day using Chilled Water Storage
A conventional chiller of 1,247.49 TR (774.99 kWe), which is higher than the existing chiller capacity, is required. Total electrical energy consumption of the chiller is 5,424.9 kWhe.

Case 04: Shifting peak cooling load only
Total peak cooling load of 1,322.0 TRh should be stored during off-peak hours using conventional chillers.

Case 06: Shifting peak cooling load
only Glycol chiller is only used for ice making and conventional chiller is used to meet the cooling load during day and off-peak hours.

Storage Calculation
Storage required to shift peak and day cooling load to off-peak is 6,403.7 TRh and to shift peak cooling load to off-peak is 1,322.0 TRh. Equation 3 is used to calculate the required storage capacity for chilled water and Equation 4 is used to calculate the required storage capacity for ice storage. where; V = TES tank volume, m 3 X = amount of thermal capacity required, ton-h L = latent heat of fusion of ice, Btu/lb SG = specific gravity, kg/m 3 eff = storage efficiency, typically 0.99 * 0.454 kg/lb ... (4) where; V = TES tank volume, m 3 X = amount of thermal capacity required, ton-h ΔT = temperature difference, °C CP = specific heat of water, Btu/lb°c ENGINEER 56 6 SG = specific gravity, kg/m 3 eff = storage efficiency, typically 0.90

Operating Cost
Energy consumption of chiller and cooling towers are considered when calculating the operating cost of the existing system as they are the main components to be replaced when implementing the TES. Table 3 below shows the energy usage as well as the applicable electricity costs during the "Off Peak", "Day" and "Peak" durations. Accordingly, the Table 4 shows the cooling tower's Operating costs for a day.

Investment Cost
The main investment is the installation cost of the equipment. Based on market prices prevailed in 2017. Costs used in this study for the financial analysis are shown in Table 5.

Benefits to the Industry
The customers will be benefited through shifting their peak time energy usage to offpeak period due to the TOU tariff offered by the CEB. Summary of financial analysis done for six cases is shown in Table 7.

Utility Benefits
Benefits to the utility can be calculated using avoided cost method. Data from system control centre of Ceylon Electricity Board (CEB) shows that usually 115MW, GT7_Gas turbine at Kelanitissa power station, 160MW, combined cycle AES Kelanitissa power station and 300MWX3, Lakvijaya coal power station are in the merit order dispatch margin during Peak, Day and Off-peak periods. Therefore, energy reduction in peak period is considered to be reduced from GT 7 and reduction in the day period is considered to be from AES power plant. Energy increase in the off-peak period due to shifted load is considered to be from Lakvijaya coal power station.
The operating costs of these power plants are shown in Table 7. Project IRR was calculated from the utility side for the above six cases expecting investment will be done by the utility.  Delay in the investment costs on new power plants that will be required if demand is not reduced.
 Delay in the investment on transmission and distribution upgrades.
 Efficiency improvement of the coal power plants during off-peak time through the increased demand and avoiding of deloading the units.
 Increase in the system stability.

Conclusions
CWS with shifting only the peak cooling load to off-peak hours is the best TES solution for the Cinnamon Lakeside Hotel because no additional chillers are required. Space limitations should also be considered when selecting a TES system. Shifting only peak cooling load to off-peak also requires less storage capacity compared to shifting both peak and day cooling loads.
Simple payback of 2.43 years and project IRR of 40.76% for case 04 are good financial indicators for a project. These figures will attract investors on TES shifting peak cooling load to off-peak hours using CWS system.
Ice storage systems will increase the temperature difference between return water and chilled water. This will reduce the chilled water flow rate required to meet the cooling load of the building. Reduction of the chilled water flow rate will reduce the pump and fan motor sizes which reduce the energy consumption and investment of the HVAC system. Ice storage system will also reduce the duct and pipe sizes due to high temperature difference achieved between chilled water and return water.
Implementing an ice storage system at the construction stage of a hotel will reduce the investment on HVAC system due to the reduction of equipment sizes, duct work and pipe diameter sizes. This would also reduce the total energy consumption of HVAC system due to reduction of equipment sizes. Implementing the ice storage system at the initial stage of construction could give high project IRR for case 06 which would attract investors.
Shifting only peak cooling load to off-peak hours at the selected hotel will reduce the utility peak demand by 205 kW. If this can be projected to 50 similar capacity buildings, utility can achieve 10 MW peak shavings.