As promised, we continue the topic of adiabatic cooling in terms of investment and operating costs.
In the last post, I showed that obtaining 1 kWh of cooling using adiabatic cooling can be up to 52.8 times cheaper than obtaining 1 kWh using a compressor. It sounds fantastic. Let's now look at the investment and operating costs (energy) for air conditioning a specific room.
Let's assume that we have a room with a total heat gain of Qc=10 kW and a moisture gain of wc=2 kg/h. We use a completely air-based ventilation and air conditioning system. The parameters of the outside air are +30°C and RH=45%. The temperature to be maintained in the room is +24°C, equal to the temperature of the air being supplied. The cooling recovery efficiency on the counter-flow heat exchanger is 80%. The internal resistance and the sum of internal resistances of the air supply section of the air handling unit is dpn=1000 Pa, and the fan efficiency is 65%. The internal resistance and the sum of internal resistances of the air exhaust section of the air handling unit is dpn=700 Pa, and the fan efficiency is 65%.
For the purpose of energy consumption calculations, we assume that air conditioning operates from June to September from 9:00 am to 6:00 pm from Monday to Friday. The climate zone is Katowice. The price of 1 kWh of electricity is 0.6 PLN. The price of 1 kWh of cooling is 0.23 PLN. The price of 1 m3 of water is 5 PLN.
Let's consider 2 variants:
Variant 1 - air conditioning using a refrigeration system with direct evaporation of the refrigerant. To maintain decent comfort, we assume a maximum temperature difference between the supply and exhaust air of dt=10K. According to the analysis on the Mollier chart, the required air volume for assimilating the given heat gains is 2840 m3/h.
The energy consumption calculations carried out using the "energy consumption calculator" that is an integral part of the IX CHART program are as follows:
So the estimated energy costs for the air conditioning system over the course of a year will be 1,877.8 zł, of which the cost of cooling energy will be 926.5 zł, while the electricity consumption for driving the fan motors will be 951.3 zł.
Variant 2 - air conditioning using indirect evaporative cooling, i.e., using a cross-flow heat exchanger. Here, we have limited ability to achieve the supply air temperature, which is limited by the wet bulb temperature for the given air parameters. We exhaust air from the room at +24°C (plus 1°C for fan heating) and 55% RH, which is then humidified. The air is cooled to +19.9°C, which is directed to a cross-flow heat exchanger with 80% efficiency, cooling the incoming air drawn at +30°C and 45% RH to a final condition of +21.9°C. So, only a possible dt of 2.1°C can be achieved. How much ventilation air is needed to remove the heat gains? According to the Mollier chart analysis, the necessary amount of air to assimilate the given heat gains is 12,058 m3/h, which is 4.25 times more than in the compressor cooling system.
What is the impact of such a large increase in the amount of air being conditioned on energy consumption? Well, you don't have to pay much for cooling alone, as it is only 47.7 PLN, but the increase in the amount of air has caused a very significant increase in the amount of electricity needed to drive the fans, which amounted to 4625.3 PLN.
In conclusion, the annual cost of adiabatic cooling amounts to 4,673 PLN and is more expensive than traditional cooling, so further cost analysis of this investment does not make sense.
When does adiabatic cooling make sense? In my opinion, only as a hybrid solution, i.e. pre-cooling using an intermediate adiabatic cooler (adiabatic cooling of the discharged air), but the final cooling should be carried out in a cooler powered by a refrigeration unit.
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