Impact of HVAC Systems on Building Peak Electricity Load

Transcription

1 Impact of HVAC Systems on Building Peak Electricity Load Janez Mandelj 1, Gorazd Krese, Matjaž Prek, Vincenc Butala Faculty of Mechanical Engineering, University of Ljubljana Aškerčeva 6, SI-1000 Ljubljana, Slovenia 1 janez.mandelj@fs.uni-lj.si Abstract The temperature-sensitivity of electricity demand is growing due to the increasing use of air conditioning. As a consequence periods of extremely high temperature, which are becoming more frequent due to global warming, result in increasingly high and unpredictable peak demand. The latter forces utilities to supply expensive electricity from costly purchased grid power since electricity can t be stored cost effectively with the current technologies. Consequently the interest in shifting cooling requirement to off-peak hours is growing. This paper examines the relationship between building peak load and electric energy consumption due to air conditioning. Four commercial buildings, of which two had ice thermal storage, were selected as case studies. The load curves of each building were compared against the load curves of the HVAC system serving it, as well as to the course of external temperature. Keywords peak load; HVAC energy consumption; storage system; airconditioned buildings 1. Introduction Commercial buildings are one of the largest users of energy, representing 30-40% of primary energy consumption in developed countries [1]. According to the EC s Joint Research Centre (2009), Heating, Ventilation and AC systems in the 27 European Union Member States were estimated to account for approximately 313 TWh of electricity use in 2007, about 11% of the total TWh of electricity consumed in Europe that year (Table 1). Table 1. EC Joint Research Center, Institute for Energy, Equipment Electrical consumption as % of total EU use in 2007 Air conditioning units and chillers 0.75 Fans in ventilation systems 3.34 Pumps 1.81 Space and Hot Water Heating 5.23

2 HVAC systems component are the largest electricity-consuming item, accounting for 40-60% of the total electricity consumption and the second largest items are electric lighting component with 20-30% of the total electricity consumption in commercial buildings. Most of the heating, ventilation, and air-conditioning (HVAC) plants in the commercial sector, especially purpose-built office buildings, operate between 07:00 and 19:00, the usual office (business) hours. The increase in electricity demand for short periods of time (a few hours in a day) is referred as peak load. There is a growing interest in shifting some of the peak load, particularly cooling requirement, to the late evening and early hours in the morning. There are two ways to shift a building`s cooling load passive or active. The former makes use of the buildings` thermal mass as a temporary thermal storage to dampen and smooth out the diurnal temperature variation, while the latter adopts ice storage systems to spread out the building`s total cooling load over a 24-hour period. Use of ice storage systems and also the other approaches for decreasing the electrical peak load was studied in [3-6]. In order to properly consider load shifting, cooling or otherwise, the end-use load pattern should be determined. In general, electricity consumption can be grouped into four major end-users, namely heating ventilation and air-conditioning (HVAC), lighting, miscellaneous power and lifts [7]. 2. Descriptions of Buildings A total of four fully air-conditioned buildings were selected for this pilot case study. In order to retain the individual building anonymity, these fore all fore buildings are referred to buildings A, B, C and D in this paper. Building A is an 21-storey office building with conditioned gross internal area (CGIA) of 19429,59 m 2. Building has seven centralized full air-conditioning system plants with a constant air volume (CAV) and fan coil unit (FCU), respectively. Office sector is served from two water cooled vaporcompression liquid chiller and twelve ice-storage banks with cooling capacity 537 kw. The building has one another water cooled vaporcompression liquid chiller for IT room sector with cooling capacity 401 kw. Building B is a shopping center with conditioned gross internal area (CGIA) of 25376,86 m 2. There are six full air-conditioning (CAV) systems for shop sales area. Office sector is served from water cooled vaporcompression liquid chiller and seven ice-storage banks with cooling capacity 714 kw. Building C is a sport center with conditioned gross internal area (CGIA) of 485,64 m 2. The building has two centralized full air-conditioning (CAV) systems and the chiller plant is air cooled vapor-compression liquid chiller. Building D is an 2-storey office building with conditioned gross internal area (CGIA) of 1437,75 m 2. The centralized full air-conditioning system

3 plant is (CAV) with fan coil units and the chiller plant is air cooled vaporcompression liquid chiller with cooling capacity 122 kw. 3. Results Hourly Load Profiles An analysis energy consumption profiles was made for the hottest day in August, presented in Fig. 1. For metering electricity consumption of HVAC component we used Landis + Gry Domestic devices. Fig. 1 Average outside air temperature in August Fig. 2 shows a typical hourly load profiles in August for Building A. The total electrical consumptions of building were compared with major electricity end-user, namely heating, ventilation, and air-conditioning (HVAC). Two major component of HVAC system are AHU and chiller. The total building electrical consumption load rose rapidly from 186,8 kwh at 04:00 to 316,8 kwh at 08:00 with a peak of 374,8 kwh at 19:00. During the office hours, the total building electrical load difference was generally within 5,9 %. The total building electrical load during the night and the early hours in the morning was generally is about 58% of the daytime peak value because of chillers operates overnight due to fill ice-storage system. HVAC accounted for 56% of the total building electrical load.

4 Fig. 2 Measured hourly load profiles for Building A The hourly load profiles for building B are shown in Fig. 3. The peak total building electrical load was 586 kwh and occurred at 14:00. During the occupancy schedule, the total building electrical load difference was 13%. The night-time load was about 34% of the peak load during the day. HVAC accounted for 34% of the total building electrical load. Fig. 3 Measured hourly load profiles for Building B

5 Fig. 4 shows the hourly load profiles for Building C. The total building electrical load peaked at 10:00 with 294 kwh. HVAC accounted for 2% of the total building electrical load. Fig. 4 Measured hourly load profiles for Building C Likewise, the hourly load profiles for building D are shown in Fig. 5. The total building electrical load had a peak of 16 kwh at 14:00. In this case HVAC accounted for 63% of the total building electrical load. Fig. 5 Measured hourly load profiles for Building D

6 Peak Electrical Consumption Thermal storage techniques could be used to reduce this peak requirement. Energy storage techniques could be used to make use of building/s structural mass as a temporary thermal store [8]. An alternative is thermal energy storage based on thermally stratified chilled water storage [9] or ice storage technology [10], in which cooling capacity is charged up during the night and used to meet the daytime peak loads. In building D the total electrical energy use of the whole building for typical working day in August is shown in Fig. 6. The maximum electrical demand was 16,3 kw at 14:00. An analysis of this peak demand was conducted. It was found that chiller plant reaches its peak of 10,8 kw at 14:00 and presented 66% of the total building electrical load. Fig. 6 Energy use as % of overall energy use in Building D for typical working day in August In building A the total electrical energy use of the whole building for typical working day in August is shown in Fig. 7, with already installed storage system, the maximum electrical demand was 374,8 kw at 19:00. An analysis showed that the first largest consumer Chiller reached its peak demand of 202 kw at 21:00 and presented 58% of the total building electrical load and the second largest consumer is AHU with its peak demand of 44 kw at 17:00 and presented 13% of the total building electrical load.

7 Fig. 7 Energy use as % of overall energy use in Building A for typical working day in August In building B the total electrical energy use of the whole building is shown in Fig. 8 with already installed storage system, the maximum electrical demand was 588,4 kw at 15:00. An analysis of this peak demand was conducted. It was found that chiller plant reaches its peak of 143,2 kw at 15:00 and AHU 43,4 kw at 15:00. In this case AHU accounted for 7% and chiller for 24% of the total building electrical load. Fig. 8 Energy use as % of overall energy use in Building B for typical working day in August

8 4. Conclusions The total of four high-rise fully air-conditioned were selected for studying the electrical end-use load characteristics. Building C was not suitable for this case study since the air conditioning area represent only a minor fraction of the total building area (i.e. less than 20%). Therefore the HVAC system has an insignificant impact on the building peak electricity demand. The total building load was broken down into major electricity endusers, namely heating, ventilation and air-conditioning (HVAC) and other end-users. The hourly load profiles of these electricity end-users were monitored during the hot summer month of August. It was found that HVAC systems with ice-storage banks, accounting for 31% to 50% of total building electrical demand during the operating time from 07:00 to 19:00 of building. The HVAC systems without ice-storage banks accounted for 40% to 62% of total building load. Electricity use peaked during the hot summer month of August. The marked seasonal increase was due mainly to the demand for air-conditioning. An analysis of the peak load reveled that chiller load was a major component. It was found that chiller load shifting using thermal chilled store could reduce the peak building load by 15%. Acknowledgment This work is performed with the support of Intelligent Energy Europe (IEE) as part of iservcmb project. The sole responsibility for the content of this document lies with the authors. It does not represent the opinion of the Community. The European Commission is not responsible for any use that may be made of the information contained therein.

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