Air-Cooled, Water-Cooled, and Geothermal Systems
As discussed in a separate article, a basic refrigeration cycle consists of four processes: evaporation, compression, condensation, and expansion. To refresh, the cycle will operate along the following steps:
Refrigerant passes through the evaporator, absorbing heat
The refrigerant, now a vapor, gets compressed into a superheated vapor
The refrigerant then passes through the condenser, rejecting heat
The refrigerant, now a liquid, expands rapidly through the expansion valve
The cycle repeats as the refrigerant enters the evaporator ready to absorb more heat
This article will focus on the methods of condensation: air-cooled, water-cooled, and geothermal. The condenser is a simple heat exchanger with the purpose being to remove heat from the refrigerant and place it into the surrounding environment. That is the entire idea of the refrigeration cycle, absorb heat from the conditioned space into the refrigerant as the spot of the evaporator and reject heat from the refrigerant to the outside.
Air-Cooled
Air-cooled systems use fans to reject heat reject heat at the spot of the condenser. In other words, the fans blow air directly over the coils of the condenser. The benefits of this system are low initial cost and ease of installation. Depending on the atmosphere, this system could be as efficient as a water-cooled or geothermal system. The downside when it comes to efficiency is that it is highly dependent on outside dry-bulb temperature. The hotter the air, the less efficient the process of condensation will be. Examples include: residential condensing units, air-source heat pumps, air-cooled chillers, mini split units, and window air conditioners.
Water-Cooled
Water-cooled systems introduce a cooling tower, condenser water loop, and condenser pumps into the equation. Instead of blowing air directly over the condenser, the condenser will sit inside of a tube. Water flows inside of that tube, absorbing heat from the hot refrigerant. The hot water is pumped to the cooling tower. There are two types of cooling towers: open circuit and closed circuit. In both types, the cooling tower will use a large fan to pull air across the water in order to cool it. In an open circuit cooling tower, the water is directly exposed to the outside air. The air will be either be sprayed into the air or run down coils along the edge of the cooling tower. By exposing the water directly to the air, the water can evaporate and cool to a lower temperature. In a closed circuit cooling tower, the water flows through an enclosed heat exchanger. The tower fan will blow air across the heat exchanger, similar to an air-cooled system. The main benefits of a water-cooled system are increased system capacity and efficiency. A cooling tower can more effectively control the temperature conditions at the spot of the condenser over an air-cooled system. As discussed in a separate article, water-cooled systems can also incorporate a condenser water reset strategy to further increase efficiency. The downside to this type of system is high initial cost and difficulty in installation. Also, water-cooled systems are only really applicable to large scale buildings. Examples include water-cooled chillers and water-source heat pumps.
Geothermal
Geothermal systems rely on permanent heat sinks to reject heat into. Similar to water-cooled systems, the condenser rejects heat into a circulating water loop. Instead of that water going to a cooling tower, the water will circulate either underground or in a large lake of water. The heat sink will be large enough to absorb all of the heat necessary to keep a steady return temperature. The return temperature of the condenser water will approach the temperature of the heat sink. This type of system is extremely efficient. It is essentially the same as a water-cooled system without the energy consumption of the cooling tower. There are difficulties in installing these types of systems. The main problem is being able to drill several hundred feet underground for the condenser water loop. City code or feasibility, such as an inner-city building, may make installation of this type impossible. However, for new construction in an area with a lot of surrounding space, a geothermal system should be seriously considered.