Ambient temperature district heating / cooling systems

A lot of people are familiar with district heating systems. Hot water or steam from a central heating plant is pumped through a network of insulated pipes to nearby buildings. Some district heating systems take advantage of heat produced by a combined heat and power (CHP) plant that greatly improves the overall operating efficiency. One of the downsides of conventional hot water or steam distribution systems is that the piping systems are expensive to install becaus they must be well insulated to reduce heat loss to the ground. Hot water / steam systems are not able to take advantage low grade waste heat sources such as

waste heat from industrial processes, refrigeration plants in ice arenas, cooling systems in server farms and other commercial buildings. 

Some district systems include district cooling with a central chiller plant with a second pair of expensive insulated pipes to deliver chilled water to nearby buildings. Central chiller plants also produce a great deal of low-grade waste heat that is typically dissipated to the atmosphere through a cooling tower. 

The main advantage of a district system that delivers hot and chilled water to its customers is that the primary mechanical equipment (the boilers and chillers) in the individual buildings is eliminated. A single central plant is more easily maintained and operated at peak efficiency than individual plants operating in each of the smaller buildings connected to it.  

An ambient temperature district heating / cooling system is kind of a hybrid district system. A GHX acts as an energy storage device for heat pumps located in the buildings connected to the system. Energy rejected from a building that needs cooling is rejected to the GHX. Buildings that require space heating or domestic hot water extract heat from the GHX. Waste heat from industrial processes can be stored in the ground, as can solar thermal energy or waste heat from a CHP plant. 

These types of system are ideally connected to a number of buildings with diverse building loads...some buildings that reject more heat to the ground, some that extract more heat than they reject. If the building loads are relatively balanced the temperature from the GHX becomes more consistent and more stable. There are several advantages to an ambient temperature system compared to a high-temperature system:

• The distribution piping does not have to be insulated...uninsulated, low-cost HDPE pipe can be used to connect the buildings to the system.
• Low-grade heat sources can be utilized to enhance the efficiency of the system (heat from refrigeration systems, process heat, solar thermal, etc.)
• The same piping system provides both heating and cooling for buildings connected to it. 
• Low-grade heat that is only available in summer can be stored in the GHX till fall and winter when it can be used. 
• Low winter time outdoor air temperatures can be used to lower the temperature of the GHX to allow the system to operate more efficiently for summer time cooling.