The predominant sectors the domestic & commercial sectors. The air conditioners (ACs) based on vapor compression cycle can be classified into:
- Room air conditioners: split and unitary air conditioners (1-3 TR)
- Duct and package air conditioners: 3-15 TR
- Central air conditioners: > 15 TR.
The majority of the air conditioners are split air conditioners (1-3 TR) and in the central segment the chillers sizes are 100-200 TR for commercial buildings and 900-1500 TR (for industrial AC applications). Most of the central machines in the large capacities are based on screw compressors.
The commonly used refrigerants for the air conditioners are:
- R134A (CH2FCF3)
- R410A (CH2F2/CHF2CF3) (50%/50%)
- R290 (propane).
While the smaller units use R 134A the larger units use R 410A.
The sectors which use the air conditioners are the domestic sector where unitary (window) and split systems are used; and the commercial sector where both split as well as the central systems are in use.
The annual sale of ACs is around 3-4 million units/year and annual sales is around Rs 30,000 crores.
The annual physical growth of the AC sector is 14%. Energy effi ciency improvement is growing at the rate of 3% which implies that the power growth is around 11%.
Cooling load and the market scenario
The two major segments or compression air conditioners are room air conditioners (1-3 TR) and central ACs. The installed capacity of air conditioners in India are nearly 22 million TR with the domestic sector accounting for nearly 15 million TR and the commercial 7 million TR. Table 1 gives the national scenario of the contribution from air conditioning segment.
It can be seen from Table 1 that the major dominance of the AC power is from the domestic sector which accounts for nearly 22% of the national power. The AC power in the commercial sector account for 9% which gives a total of 31% of the national share of the power. However, when it comes to energy the contribution from ACs is quite low (hardly 10%) because of the low load factor of the ACs. While the power ratings of ACs are high their load factors are low due to seasonal usage, weather conditions and non continuous use only during certain hours of the day.
In the domestic sector 70% of the ACs are unitary (window) & the balance of split. In the commercial sector the break up of unitary & split are 50% each (See Figures 1 & 2).
The break up of the room and central ACs in commercial sector are given in Fig. 3. Among the central air conditioners nearly 28% are in the range of 100-200 TR and used for commercial building load and nearly 26% are of 900-1500 TR and used for industrial air conditioning applications. The balance air conditioners come in varying sizes from 50 TR to 2000 TR.
Fig. 4 shows the load variation with months for a commercial building in Delhi. It is clear that the variation is from 60% in winter to almost 160% of the average value during summer months. This can be attributed to AC loads.
However, on the overall load scenario of the various regional grids, the winter and summer loads do not show any perceivable differences. This means that the AC loads do not impact the national grid significantly in terms of additional loads or load peaks. Figures 5-9 show the average load during summer and winter for the various national regions of the grid. Except in the Eastern and Southern regions, the peak due to AC load does not get reflected in either the peak load or average load for the various regions. In the eastern region and in 2014 in the southern region, the average load is higher in summer indicating increased use of AC.
Solar photovoltaic (SPV) powered AC plants integral with cold thermal energy storage (AC-CTES) using ice build and melt processes
In tropical countries like India, the SPV power generating period can be classified into three distinct phases:
- Summer season with low stochastic losses, high, regular and reliable incident solar radiation.
- Winter season with medium level of regular and reliable incident radiation.
- Rainy season characterized by high stochastic losses (due to cloudy and rainy weather) and low reliability.
Solar photovoltaic systems can provide the power for central AC plants as fossil fuel substitution options. One of the advantages of SPV power is it coincides with the commercial AC loads implying that very less electric storage is required for off grid operation. In a typical PSV plant in India the power output is 0.068 kW/m2 of SPV panel area. The daily energy generation is 0.885 kWh/m2. Approximately 6 m2 of panels are required for providing 1 kW peak power. For providing 1 kW of average power over 12 hours, then nearly 15 m2 of panels are required. Figures 10 & 11 give the curves for a typical 100 kW AC plant. The SPV peak capacity is around 160 kW. This includes not only the compressor power but also the balance of plant such as pumps, cooling tower fans, AHU fans, etc.
For a typical 100 kW system the capital cost of SPV (combination of crystalline silicon and thin film to handle the Indian weather conditions) power plant (together with the inverter, batteries, etc.) is around Rs 73 lakhs and the saving in energy is around Rs 8,760/day which gives a pay back period of 5.5 years considering an energy charge of Rs 12/kWh. Presently, many SPV systems get paid as much as Rs. 8-10/kWh in the grid tied mode due to the renewable energy obligation. For smart grid configurations SPV lends itself an ideal candidate energy source because AC with cold thermal energy storage CTES referred to as AC-CTES is a means of balancing the mismatch between the load curve and the solar generation curve. The peak power input is critical in SPV powered plants where the plant capacity directly determines the maximum power and energy generation.
In the operation of AC-CTES with SPV the power input is reduced and also the energy consumption is lower as compared to conventional AC operation. In grid power operation, the non-peak period can be used for ice free processes.
SPV power is available as a parabolic output and only for 12 hours in a day. For providing electrical power in the nonsunshine period, electrical energy storage is required in the form of battery banks. The energy efficiency of electrical energy storage is 80% which calls for 20% additional generation to meet the load. If the AC-CTES system is available, the storage of electrical energy in battery banks is totally avoided. Further during the sunshine period, the excess energy can be used for the ice freeze process. During the non-sunshine period the ice melt process will provide the cooling effect. With AC-CTES systems, the chilling (cooling effect generation) and the cooling effect utilization are de-coupled. As a result only those equipment associated with chilling are used during the ice freeze process and equipment with AHU operation will be used in the ice melt process. Thus the total power at any time of operation is much lower than the simultaneous chilling and AHU operation of conventional ACs. Also, the CTES can be modulated as per solar incident radiation. Table 3 gives a comparison of AC-CTES with the conventional systems.
The design criterion are:
- Avoiding battery storage of SPV power altogether.
- Total use of SPV power during the sunshine period for providing cooling to the extent required and the storage of the balance through the ice storage system for use during the non-solar period.
AC-CTES based on SPV can be operated through summer & winter (depending on the need) & with limited applicability during the rainy season.
New commercial and domestic buildings can go in for SPV based AC-CTES.
Conclusions
The conclusion of the study are as follows:
- AC loads account for nearly 16.5% of the total national power. Energy wise, AC loads account for 9.8% of the total energy.
- The growth rate of AC power is 14% /year but considering the 3%/year decrease in power due to energy effi ciency, the annual growth in power will be only 11%.
- Switchover to solar photovoltaic based AC systems is called for in green building concepts. The parabolic nature of solar energy requires cold thermal energy storage which can be in the form of ice build and melt systems.
