Why hybrid options make sense

Most people considering a GCHP system for their home or business have questions about the return on their investment. Will they save enough in energy cost to pay for the difference in cost between a conventional HVAC system and a GCHP system. If we, as designers and contractors can find ways of installing a system for less money and/or reduce energy costs more, we can improve the return on their investment. Hybrid options for a system can reduce the first cost... 

with little or no impact on the energy cost, and in some projects can even actually reduce overall energy cost of the system. The most common hybrid options in a GCHP system are a fluid cooler or closed circuit cooling tower and a boiler. A fluid cooler can be used in different ways. A common method is to simply use a fluid cooler to dissipate excess heat from a GCHP system at times of peak cooling. A fluid cooler can also be used during off-peak times to pre-cool a GHX, either during cool nights, or on a seasonal basis during the winter. This option allows the designer to specify a much smaller fluid cooler because cooler night time or winter air temperatures greatly increase the capacity of a fluid cooler. 

Auxiliary boilers are often used to increase the temperature of a building loop when the GHX temperature drops lower than desired, reducing heat extraction from the ground. Both of these options, when used effectively, reduce the imbalance of energy transfer to and from the ground and can greatly reduce the size and cost of a GHX. They are often used in projects where the land area available for the construction of a GHX is limited. 

The graph illustrates why using auxiliary heat or auxiliary heat dissipation device does not penalize the performance of a GCHP system greatly. An auxiliary boiler and fluid cooler are shown on the annual energy load profile for this project. The boiler is designed to provide 40% of the peak heating load and the fluid cooler 40% of the peak heat dissipation. The dashed lines above the solid lines indicate the peak heating and cooling provided by the fluid cooler and the boiler. The dark red and blue areas indicate the percentage of heating and heat dissipation the devices actually provide...less than 3% in either heating or cooling. 

The 2nd and 3rd graph show daily cooling load profiles for July 31 (the design day for this project) and for August 1. The light blue areas indicate the amount of peak cooling provided by a fluid cooler in relation to the heat absorbed by the GHX. Even on July 31, the design day, only a small percentage of the total heat is dissipated by the fluid cooler, and on the following day, it only operates for one hour. There are only a small number of peak hours the fluid cooler, or an auxiliary boiler is actually needed. The size of the GHX, however, can often be reduced by 15% to 30% using these devices, and can provide the owner a much better return on their investment.