Watch the pumping power!

The efficiency of a GCHP system is more than selecting the most efficient heat pump. The design of the GHX and the distribution system can have a major impact on the overall system efficiency. Some of the GHX details that have an effect on the system efficiency include:

• Detailed GHX configuration
• Heat transfer fluid selection
• Mechanical system configuration

This article we'll consider the impact the configuration of the GHX has on the overall system efficiency.

A vertical GHX for a 30 ton (105 kW) system can be configured many different ways. The size of the GHX circuit piping, the SDR number, the number of GHX circuits connected to a supply and return runout pair, the configuration of the supply and return headers, the pipe size used for the supply and return runout pairs, the heat transfer fluid mixture and even the temperature the fluid in the GHX is designed to operate at. 

Most heat pump manufacturers recommend a flow rate of approximately 3 gpm per ton (0.054 l/s per kW). The maximum flow rate through the GHX is often based on the maximum flow rate required for the equipment installed. (there are a number of other factors that should be considered in determining the flow rate through the GHX...they will be discussed in future articles). For this discussion the GHX will be configured to operate at a flow rate of 30 tons x 3 gpm, or 90 gpm (105 kW x 0.054 l/s = 5.67 l/s)

Calculations to determine the total length of borehole needed for the project indicate  approximately 5,300' (1,615 m) of borehole are needed if we specify 1" pipe (32 mm metric pipe) or 5,080' (1,548 m) of borehole if  1.25" pipe (40 mm metric pipe) is specified. Discussions with local drillers have determined the most cost-effective depth to drill to in the area is 350-450' (106-137 m). The location of the borehole field is approximately 165' (50 m) from the mechanical room. How the GHX is configured can have a major impact on overall system efficiency.

Option 1 is a GHX field with a total of 12 boreholes to a depth of 441' (134 m) using 1.00" pipe (equivalent to 32 mm metric pipe). Option 2 is a GHX field with a total of 15 boreholes to a depth of 340' (103 m) built with 1.25" pipe (equivalent to 40 mm metric pipe). The pressure drop for GHX 1 is 58.7' (175 kPa) and will require approximately 1.9 hp (1.4 kW) pump to circulate fluid through the GHX, 

The second GHX option with 15 boreholes and larger GHX circuit pipe has a much lower pressure drop.  Pump power is reduced to approximately 0.75 hp (0.55 kW)...a reduction of 61%. Depending on how the mechanical system schematic is designed and how the pumps are controlled, this can have a significant impact on the overall system performance.