Commercial Geothermal Heat Pump Systems (Part 2)
Last May, we began a discussion of commercial applications for ground source heat pumps (GSHP). A confluence of interest in renewable energy, LEED accreditation, government subsidies, and even smart grid technology underlie a swelling interest in this technology.
Although multi-unit GSHP systems have been deployed for more than two decades, there are several new hydronic technologies that can now be implemented to further improve the operating characteristics of these systems.
A typical multi-unit closed-loop GSHP system is shown in Figure 1.
This approach sustains flow through all heat pumps 24/7, regardless of whether any given heat pump happens to be operating. Although simple in concept, this approach wastes considerable pumping energy, and, thus, increases operating cost.
The wattage associated with maintaining flow through an individual heat pump unit can be estimated with the following formula:
- we = electrical wattage into circulator to supply flow
f = flow rate through heat pump unit (gpm)
∆P = pressure drop of heat pump and piping components connecting it to the mains (psi)
nw/w = wire-to-water efficiency of the loop circulator (decimal %)
For example: Assume a nominal 4-ton water-to-air heat pump unit operates at a set flow rate of 9 gpm. Its water-side heat exchanger has a pressure drop of 2.6 psi under these conditions. The heat pump is connected to the system mains with 1-inch size piping and fittings that have a total equivalent length of 40 feet of 1-inch copper tube. The pressure drop of the latter at 9 gpm (with 40ºF water) is 0.84 psi. Thus, the total pressure drop of the heat pump plus piping is 3.44 psi. Assuming the loop circulator has a wire-to-water efficiency of 40%, the electrical power required to maintain flow through this unit is:
Thus, if this scenario represents the average condition in a system with 50 heat pumps, the annual cost of driving flow through inoperative heat pumps is $833/yr.
Furthermore, this parasitic energy ends up as heat gain to the building. This is not necessarily bad in winter, but it certainly increases cooling load, and, thus, produces even higher net electrical cost.
Waste Not Thy HeadWhy not stop flow through heat pumps when they are not operating? In the past, the concern may have been differential pressure control, which would have required a differential pressure valve or a circulator equipped with VFD drive for variable speed control. Today this task can be handled, at least in small- to moderate-size systems, by a self-contained, variable-speed, pressure-regulated circulator using ECM (electronically commutated motor) technology. The concept is shown in Figure 2.
This system equips each heat pump with a standard, electrically operated zone valve, which opens only when the heat pump is operating. When a given zone valve closes, the pressure-regulated circulator immediately senses an attempt to shift along its current pump curve to a slightly higher differential pressure. Its internal logic quickly counters by decreasing motor speed to hold the prescribed differential pressure.
When a two-pipe reverse return distribution system is used, a proportional differential pressure control mode for the circulator is likely the best option. An example of how this algorithm works is shown in Figure 3.
This operating mode accounts for a decreasing pressure drop along the mains as fewer heat pumps operate. By decreasing differential pressure with decreasing flow, this algorithm accounts for lower flow through branches, as well as along the mains.
Two-Sided HydronicsThere are lots of commercial HVAC applications that require low- to medium-temperature heated water or chilled water. Examples of the first include radiant panel heating, pool heating and domestic water preheating. Possibilities for the latter include chilled water air handlers, chilled beams and radiant cooling. In some cases one of these heating loads may even operate simultaneously with one of the cooling loads.
Such applications are well matched with a multiple water-to-water (w/w) heat pump system combined with a ground loop. In some cases, w/w heat pumps are sold as “heating only” units. Such units have no reversing valve. Units equipped with reversing valves can easily switch between heating and cooling as the load requires.
In heating-only applications the ground loop supplies heat to the heat pump evaporators. In cooling-only applications, it absorbs heat from the heat pump condensers. In simultaneous heating/cooling situations the ground loop intercedes as a buffering medium, providing the “net” heating or cooling capacity needed.
One example of a multiple water-to-water heat pump system is shown in Figure 4.
Although this system will work as shown, it uses significantly more pumping energy compared to other currently available alternatives. One of those alternatives is shown in Figure 5.
Here, each w/w heat pump has two zone valves, one in the evaporator circuit and the other in the condenser circuit. Both valves open when the heat pump operates.
The evaporator and condenser circuits connect to low-flow-resistance headers. By keeping the header flow resistance low, (sizing for flow velocities of 2 ft/sec), the vast majority of the head loss in the circuit occurs across the heat pumps. If all the heat pumps are identical, the evaporator and condenser circuits are both self-balancing. Balancing valves would not be needed, although isolation valves on both sides of each heat pump are recommended. If longer headers are required, reverse return piping should be used to preserve the self-balancing characteristic.
The heat pumps are operated by a staging controller – just like that used to operate an on/off multiple boiler system. This controller is tasked with keeping the temperature in the buffer tank within a specified range. The temperature and temperature range differential could be fixed or based on outdoor reset control. The latter allows the possibility of operating the heat pump at the lowest condenser temperatures compatible with the current heat load, and, thus, maximizing their coefficient of performance.
The staging controller would also periodically rotate the operating order of the heat pumps to keep the cumulative run hours on each unit about the same.
The buffer tank provides mass to stabilize the system against small zone loads. It also serves as a hydraulic separator between the condenser circuit and the zone loads.
The earth loop is powered by a variable speed, ECM-based circulator. In this case, flow is controlled based on maintaining a set temperature drop across the earth loop. As heat pumps turn off and the evaporator load decreases, so can flow through the earth loop. Although a single circulator is shown, a lead/lag arrangement similar to that shown in Figure 1 is certainly possible.
Another design concept is shown in the schematic of Figure 6.
Here, two, 2-stage w/w heat pumps supply heat to a buffer tank. Copper coils inside this tank preheat domestic water. Depending on the temperature of the tank and the required delivery temperature for the water, the majority of the domestic water temperature rise can be accomplished across these coils. The auxiliary water heater “tops off” the temperature as required. Variable-speed circulators, in combination with zone valves, are used on the distribution system, as well as both the evaporator and condenser sides of the heat pump array. Another heat source, such as a boiler or solar collector array, can also supply heat to the buffer tank if required.
There are certainly more possibilities for using w/w geothermal heat pumps in commercial systems. In all cases, state-of-the-art hydronic technologies such as variable speed circulators, hydraulic separation and multi-stage heat production can put a final polish on the inherent efficiency of such heating and cooling devices.