How does adiabatic cooling / evaporative cooling work?

1. Factors influencing the achievable cooling effect

In indirect evaporative cooling, this water evaporation takes place on the exhaust air side of an air handling unit, whereby warm outside air is cooled via a subsequent heat recovery system. The achievable cooling of the outdoor air therefore depends on the amount of water evaporated on the exhaust air side, as well as the design and efficiency of the heat recovery system used. The exhaust air can be humidified to near saturation without causing an increase in humidity in the supply air.

In addition to the air velocity at which the evaporative cooler is flowed through, the amount of water evaporated and thus the cooling achieved depends on the air conditions with which the exhaust air enters the evaporative cooler.

The decisive factors here are:

• the air temperature prior to evaporation: the cooler it is, the less moisture it can absorb and the lower the cooling effect
• the air humidity prior to evaporation: The more water the air already contains, the less moisture it can absorb and the smaller the resulting temperature reduction

The theoretical limit of evaporative cooling is reached when the air is completely saturated with water – i.e. at a relative humidity of 100%. In HVAC systems, increases in humidity to levels of 92 to 95%, depending on the design of the evaporative cooler used, are realistic at an economically viable cost.

2. Energy savings through indirect evaporative cooling in an HVAC system

A building’s sensible cooling energy demand is primarily determined by incident solar radiation as well as internal heat loads from people, equipment and lighting systems. To maintain the permissible indoor air humidity, additional latent cooling energy is required depending on the outdoor air conditions and internal sources of moisture.

Indirect evaporative cooling is suitable for the sensible cooling of the supply air. Latent cooling required for dehumidification, or sensible cooling exceeding the potential of evaporative cooling, must still be provided by a mechanical, but appropriately smaller-sized, chiller. With a suitable system design, indirect evaporative cooling saves significantly more electrical drive energy for mechanical refrigeration than is required to overcome the additional air-side pressure drop caused by the exhaust fan.

If, as early as the planning stage, one wishes to know how much energy can be regeneratively generated and actually saved through indirect evaporative cooling, this can be determined by means of a simulation calculation for the operation of the HVAC system at the respective building site. This simulation must incorporate all outdoor air conditions occurring over the course of the year as well as the relevant design parameters of the HVAC system.

3. Calculation example using a simulated HVAC system

Simulation of an HVAC system with indirect evaporative cooling. The energy contribution of indirect evaporative cooling (adiabatic cooling) will now be illustrated using an example simulation calculation for a model building. This means that, using local meteorological data, the total cooling capacity required to cool the model building is calculated, along with the contribution made by indirect evaporative cooling over the course of the year. The results can then serve as a realistic basis for correct system sizing and for assessing the cost-effectiveness of this efficiency measure during the system planning phase.

Design parameters for a model building

The simulation was carried out for the structural configuration of the air-handling unit shown in Fig. 1, using the temperature profiles and parameters for the cooling scenario shown in Fig. 2. The system operates with summer compensation of the room air temperature and a gradual reduction of the supply air temperature. Heat recovery is modelled using a plate heat exchanger without moisture transfer from the exhaust air side to the supply air side and without any air leakage. The ratio between supply air and exhaust air flow rates is assumed to be 1:1.

The following additional design parameters relevant to the system simulation were assumed:

Air flow rate of the AHU: 52,500 m³/h

Days of use per week: 7 days

Start of daily operating hours: 6:00 am

Increase in humidity in the room: 1.0 g/kg

Minimum/maximum room humidity: 40/65% r.h.

Switching differential for evaporative cooling: 1.0 K

Humidification efficiency: 94%

Heat recovery efficiency: 0.75

The total annual energy contributions are obtained by summing the individual results determined by the simulation for each hour of the year. The calculations are based on statistical location data from the global meteorological database Meteonorm Version 6.1 for the five locations Berlin, Munich, Stuttgart, Vienna and Bregenz.

Discussion of the simulation results

The simulation clearly illustrates the cooling work performed over the course of the year and its distribution between mechanical refrigeration, indirect evaporative cooling (adiabatic cooling) and heat recovery. The reduction in load achieved by heat recovery from the building’s exhaust air alone is correspondingly low, even at the selected heat recovery coefficient of 0.75, due to the small usable temperature difference during cooling operation. However, if the exhaust air is further cooled by indirect evaporative cooling (adiabatic cooling), this leads to a significant increase in its energy contribution.

The simulation results, based on data sets for typical summers, show the average energy contributions achievable over many years of system operation, which is why they are suitable for assessing the achievable energy savings through indirect evaporative cooling and its cost-effectiveness. When considering the widely varying outdoor air conditions throughout the year, it quickly becomes clear that the refrigeration systems must deliver adequate cooling capacity under all prevailing air conditions. Therefore, the simulation results based on the extreme values for hot summers should be used for system sizing. If future climate developments are also to be taken into account, model simulations can be carried out using future meteorological data sets, provided that these are sufficiently representative.

Berlin:

Qₖ (32°C, 40% rel. hum.): 321 kW | Operating hours: 912 h/a | Qₖ, Total: 560 kW | Qₖ, Mechanical: 351 kW | Qₖ, Evap. + WRG: 209 kW | Wₖ, Total: 120,098 kWh/a | Wₖ, Mechanical: 53,887 kWh/a | Wₖ, Evap.: 56,479 kWh/a | Wₖ, WRG: 9,733 kWh/a | Reg: 55.1%

Munich:

Qₖ (32°C, 40% rel. hum.): 321 kW | Operating hours: 758 h/a | Qₖ, Total: 371 kW | Qₖ, Mechanical: 269 kW | Qₖ, Evap. + Heat recovery: 102 kW | Wₖ, total: 93,628 kWh/a | Wₖ, mechanical: 44,849 kWh/a | Wₖ, evaporative: 42,871 kWh/a | Wₖ, heat recovery: 5,909 kWh/a | Reg: 52.1%

Stuttgart:

Qₖ (32°C, 40% rel. hum.): 321 kW | Operating hours: 1,127 h/a | Qₖ, Total: 486 kW | Qₖ, Mechanical: 289 kW | Qₖ, Evap. + Heat recovery: 197 kW | Wₖ, Total: 154,993 kWh/a | Wₖ, Mechanical: 67,192 kWh/a | Wₖ, Evap.: 72,132 kWh/a | Wₖ, Heat recovery: 15,669 kWh/a | Reg: 56.6%

Vienna:

Qₖ (32°C, 40% rel. hum.): 321 kW | Operating hours: 1,213 h/a | Qₖ, Total: 598 kW | Qₖ, Mechanical: 416 kW | Qₖ, Evap. + Heat recovery: 182 kW | Wₖ, Total: 184,584 kWh/a | Wₖ, Mechanical: 96,235 kWh/a | Wₖ, Evap.: 72,873 kWh/a | Wₖ, Heat recovery: 15,477 kWh/a | Reg: 47.9%

Bregenz:

Qₖ (32°C, 40% rel. hum.): 321 kW | Operating hours: 727 h/a | Qₖ, Total: 538 kW | Qₖ, Mechanical: 364 kW | Qₖ, Evap. + heat recovery: 174 kW | Wₖ, total: 111,707 kWh/a | Wₖ, mechanical: 66,994 kWh/a | Wₖ, evaporative: 38,741 kWh/a | Wₖ, heat recovery: 5,972 kWh/a | Reg: 40.0%

The simulation results refer to the example HVAC system at 5 selected locations. The energy contribution of indirect evaporative cooling significantly reduces the cooling capacity required from the mechanical chiller for building cooling.

QK (32°C, 40% RH): Total cooling capacity under standard outdoor air conditions

QK, total*: Total cooling capacity (extreme value)

QK, mechanical*: Mechanical cooling capacity (extreme value)

QK, evap. & HRV*: Regenerative cooling capacity (extreme value)

WK, total: Total annual cooling energy output (average value)

WK, mechanical: Energy contribution of mechanical cooling (average value)

WK, evap.: Energy contribution of indirect evaporative cooling (average value)

WK, WRG: Energy contribution of heat recovery (average value)

ηReg: Renewable contribution (average value)

As the simulation shows, indirect evaporative cooling leads to significant renewable contributions. However, with the system design otherwise remaining the same, there are significant differences arising from the respective weather data of the selected locations. In regions with higher outdoor humidity – where more dehumidification is therefore required – its energy contribution shows lower relative values. This is clearly evident in Bregenz, which is correspondingly influenced climatically by its direct location on the eastern shore of Lake Constance. The total renewable share is calculated as the sum of the energy contributions from indirect evaporative cooling and heat recovery. At the selected building sites, this accounts for between 40 and 56.6% of the total annual cooling energy required.

The question of cost-effectiveness

In practice, the biggest hurdle to the use of renewable energy is cost-effectiveness. Efficiency measures such as indirect evaporative cooling (adiabatic cooling) must be cost-effective. Any additional investment costs must be recouped through the savings achieved during operation. This cost-benefit analysis must be carried out for each individual building. A reliable system simulation makes the interrelationships transparent and enables a realistic comparison with conventional building cooling measures.