ASHRAE Journal - October 2009 - 26

Variable Flow System EER Cooling Capacity (TC) 30.7 33.7 36.4 36.2 36.7 35.8 33.7 32.7 29.8 27.6 Unit EER 26.7 26.9 26.5 24.2 22.3 19.8 16.9 14.8 12 10.1 Fluid EWT 30 40 50 60 70 80 90 100 110 120 Adjusted TC 29.7 32.7 35.4 35.2 35.7 34.8 32.7 31.7 28.8 26.6 Unit Watts 1150 1253 1374 1496 1646 1808 1994 2209 2483 2733 Fan Watts 287 287 287 287 287 287 287 287 287 287 Pump*, † Watts 195 195 195 195 195 195 195 195 195 195 Total Watts 1632 1735 1856 1978 2128 2290 2476 2691 2965 3215 System EER 18.2 18.9 19.1 17.8 16.8 15.2 13.2 11.8 9.7 8.3 Variable Flow System COP Heating Capacity (TH) 24.1 28.0 31.6 35.5 39.6 43.0 46.6 Unit COP 3.76 4.28 4.72 5.18 5.63 5.97 6.31 Fluid EWT 30 40 50 60 70 80 90 Adjusted TH 25.1 29.0 32.6 36.5 40.6 44.0 47.6 Unit Watts 1878 1917 1962 2008 2061 2110 2164 Fan Watts 287 287 287 287 287 287 287 Pump*, † Watts 195 195 195 195 195 195 195 Total Watts 2360 2399 2444 2490 2543 2592 2646 System COP 3.1 3.5 3.9 4.3 4.7 5.0 5.3 Assumptions: Air cfm/ton = 400; Static pressure = 0.5; Fan efficiency = 50%; Motor efficiency = 80%; gpm/ton = 2.5; Total dynamic head = 65; Pump efficiency = 60%; Motor and drive efficiency = 90%. * Pump power is increased to compensate for flow control valve, motorized valve and strainer. † 10 ft (28 980 Pa) is added to the system head to accommodate unit pressure drop variation and pressure control setpoint. Table 1: Heat pump system EER and COP calculations. Basic High-Efficiency Designs GCHP systems operate at their greatPumping Watts With 60% Efficiency Pump Pumping hp/100 tons Grade Input/ton est efficiency when unitary equipment is Minimum Maximum used within the system,2 i.e., individual A – Excellent – 47.5 50 or Less 5 or less ground loops and circulators for each heat pump (Figure 3) and subcentral 50 – 75 5 – 7.5 B – Good 47.5 71.3 header/piping systems. In these arrange75 – 100 7.5 – 10 C – Mediocre 71.3 95.0 ments, the building is broken down into 100 – 150 10 – 15 D – Poor 95.0 142.6 smaller loops serving several heat pumps (Figure 4). Kavanaugh, et al., 2 have F – Bad 142.6 – >150 >15 promoted these concepts for a number Pump heads are calculated at 2.5 gpm/ton. For 3 gpm/ton (0.15 L/s · kW for 0.18 L/s · kW) reduce values by 17%. of years with limited traction. These strategies reduce inside loop Table 2: Pumping power benchmarks.2 piping cost and pumping horsepower. However, no diversity connection exists that allows systems Subcentral GCHP systems suffer from pumping complexin the same building to share capacity. Each system must be ity. Flow throughout the system is variable depending on individually designed and sized. Fully eliminating the build- the number of units in operation. The power requirements at ing circulation piping reduces the inside piping and circulation each circulator must be evaluated when the system is in full pumping cost. However, the well drilling cost is increased. operation. The pumps installed within these systems often Adding more bores around the building perimeter is some- have low flow and high head performance requirements. times a problem in terms of construction sequencing and When individual pumps are in operation, the flow through the installed cost. individual heat pumps is significantly beyond design needs, 26 ASHRAE Journal ashrae.org October 2009 Allowable Pump Head (ft)
ASHRAE Journal - October 2009

Table of Contents for the Digital Edition of ASHRAE Journal - October 2009
