2.7 Ground Source Heat Pumps
Sometimes referred to as geothermal systems, ground source heat pumps (GSHP) couple the condenser water side of the heat pump to a ground source. Possible couplings include vertical wells, horizontal wells or a body of water such as a pond or lake. In closed loop systems, water or an antifreeze solution is circulated through a series of pipes, while in open loop systems, water is pumped through the system and discharged openly.
The temperature of the heat sink available to the GSHP is going to be highly site dependent, and should be based upon the anticipated temperatures that will result after the system has been operational for a reasonable period of time. In some cases, a large lake may have fairly stable temperatures that can be anticipated on a monthly basis based upon historical trends. In other cases, a smaller heat sink such as drilled wells may be subject to localized temperature increases in the areas surrounding the wells, and this will need to be factored in. In addition, sites that have underground aquifers will see different resulting ground temperatures than sites without. As a result, local conditions will need to be factored into the ground temperature assumptions which are key to GSHP modeling.
Modeling of the GSHP systems will begin with the basic zoning of the building. It is common to use fairly small (5 ton and under) systems, so each will typically require a separate zone in the model. In cases where zones have the same GSHP installed and are thermally similar, combining of zones is acceptable. Some large applications of this technology will utilize multiple zone air handlers, in which case conventional zoning procedures will apply. Input descriptions for the GSHP will include heating and cooling capacities, the heating COP and cooling EER, as well as the supply fan and possibly return fan information. On some larger systems, economizers may be included.
The coupling of the GSHP system will occur through the condenser water loop, so the next step is to define this loop. Included in this definition will be information such as the pumping flow rate of the system, type of pumping (variable speed, constant speed), pump motor size as well as whether the GSHP condenser coils include three-way or two-way valves to control flow.
In cold climates, a backup boiler may be included as part of the condenser water system to prevent low water temperatures in the loop. The basic boiler information will need to be specified, as well as a minimum loop temperature which will dictate when the boiler is activated.
The final step is to define the actual ground or pond heat sink. This may include defining a schedule of monthly temperatures to represent a large body of water, although on smaller bodies of water changes in
temperature from the GSHP system will need to be accounted for. In the case of a vertical or horizontal well system,information including the type of well, well spacing and well depth will need to be included.
The baseline building will not have a GSHP.