Geothermal heat pump systems generally fall into two categories: central systems, using fewer but larger heat pumps typically making hot and chilled water to feed to conventional air handlers, and decentralized systems, which use smaller unitary heat pumps.
This decentralized system category represents the majority of the applications here in the U.S. and is the focus of this review. Unfortunately, we see many decentralized geothermal heat pump systems with fluid distribution system designs that significantly reduce overall system effectiveness and energy efficiency. It makes little sense to invest in a geothermal heat pump system to allow the net energy savings to be consumed by excess pumping energy. The ASHRAE guideline2 for geothermal pumping energy is an excellent place to start in the design process and provides a report card grade for the amount of pumping horsepower required per 100 tons of cooling; 5 hp or less merits an “A”, whereas 10 to 15 hp is a “D.” The evaluation of pumping energy should be a standard part of the design process as we consider the configuration of the fluid distribution system, the pressure drop related to pipe sizing, and the geothermal heat exchanger design.
Pumping energy also affects the amount of heat rejected to the geothermal heat exchanger. This additional cooling load can be quite significant. A recent study3 illustrated that waste heat from pumping can add cooling load to the geothermal heat exchanger on the order of 1.5% to 16% of the total heat rejected.
CENTRALIZED VS. DISTRIBUTED PUMPING
In general terms, unitary geothermal heat pump systems having fluid distribution systems applied in larger buildings fall into two categories: centralized or distributed pumping. Centralized pumping applies a central circulating pump (Figure 1) that circulates the fluid through each of the heat pumps and the geothermal heat exchanger. ASHRAE 90.1 indicates that if this pump is 10 hp or greater, a variable volume pumping solution is required, which would include two position (open/closed) valves at each heat pump and a VSD at the circulating pump, often controlled by a sensor that allows the pump to maintain a constant differential pressure across the supply and return piping. Ideally, the system would also allow a reset of the differential pressure setpoint based upon actual system load. An advantage of this configuration is the low number of circulating pumps and their central location for maintenance and service.
Additionally, larger pumps in this application will have higher wire-to-water efficiency as compared to smaller distributed pumps; however, they generally do not operate below 25% of maximum flow due to limitations on driving the pump motors at these low speeds.
Distributed pumping shifts the circulating pumps from a single location to each of the individual heat pumps (Figure 2). Each circulating pump is then sized for the pressure drop through the individual heat pump, the distribution piping, and the geothermal heat exchanger. Don Penn, P.E., CGD, president of Image Engineering Group, Ltd. in Grapevine, TX, has applied this concept with great success on more than 200 school projects since 1992. Don notes that this concept has several advantages over centralized pumping schemes.
• Circulating pumps only operate when the heat pumps are on. The best energy management strategy is to turn things off when you don’t need them.
• The control concept is very simple.
• The net pumping energy is often lowest with this approach.
• A recent retrofit project converting from central to distributed pumping is showing a 20% reduction in HVAC energy use.
Another advantage of the distributed pumping configuration is the ability to reduce the system flow rate to a single geothermal heat pump, whereas the central pumping scheme is limited by the lower limit (often 25%) of the VSD and the pump motor.
The study4 cited above also noted that the actual distributed pumping configurations projects submitted averaged pumping horsepower of 6.6 hp/100 tons, whereas the average centralized pumping configurations were 13.5 hp/100 tons — double the pumping energy of the distributed pumping projects.
HYBRID GEOTHERMAL HEAT EXCHANGERS
Numerous studies have indicated that the lowest life-cycle cost for geothermal heat pump systems can typically be achieved through the application of a “hybrid” configuration, in which an auxiliary heat sink or source device (fluid cooler or boiler) is added to the design.
In a typical commercial or institutional facility, the annual heating/cooling load balance is often cooling dominated, meaning that more heat is rejected to the ground than is removed during the heating season. In these applications, the cooling load drives the geothermal heat exchanger size, requiring more heat exchanger surface area than needed for the heating load. The result is a geothermal heat exchanger that is potentially too large for the site or exceeds the project construction budget.
The typical hybrid approach for a cooling-dominated load is to apply a closed-circuit fluid cooler in series with the geothermal heat exchanger (Figure 3). The most common control method is to turn the fluid cooler on if the geothermal loop temperature exceeds a temperature setpoint (for example, 85°F) and continue to operate until the loop temperature drops below this setpoint. Note that this configuration can only reject heat if the outside air drybulb or wetbulb temperature is lower than the fluid temperature. This approach can allow significant geothermal heat exchanger size reductions, often nearing 50%, and, if carefully applied, this configuration can approach the energy efficiency of a full-size geothermal heat exchanger.5
While this approach is very effective at reducing the first cost of geothermal heat pump systems there are some potential opportunities to make this approach even more efficient and to further reduce the system first cost. The typical hybrid configuration will allow the geothermal heat exchanger to reach a point of “thermal saturation” before the heat rejection device is engaged. This means that once the fluid temperature has reached the design setpoint, most of the heat rejection is shifted from the geothermal heat exchanger to the heat sink (fluid cooler). Unfortunately, this typically occurs when the fluid cooler is facing the least efficient conditions (high outside air drybulb and wetbulb temperatures) to reject heat and most likely during daytime on-peak electric rate periods.
Additionally, fluid cooler heat rejection is tied on a real-time basis to the operation of the heat pumps — it only rejects heat when there is a cooling load. Thus it removes one of the significant advantages of geothermal heat sinks: the ability to absorb and “time shift” thermal energy.
Depending upon the geology of the project site, the long-term average temperature of the geothermal heat exchanger may “creep” if each year more thermal energy is pushed into the earth than is removed. In simple terms, this means that each year the average temperature in the ground may be slightly higher than the year before, until some level of thermal equilibrium is reached. This temperature creep is highly variable and greatly dependent upon groundwater movement — less groundwater movement equals a greater potential for the geothermal heat exchanger to “warm up” over time. A good example of this is the Drake Landing Project in Alberta, Canada where granite geology (minimum groundwater movement) is allowing a solar collection system to achieve temperatures nearing 175° in a borehole thermal energy storage system. In this case, it is possible to drive geothermal heat exchanger temperatures to high levels.
Unfortunately, it is very difficult to determine the actual groundwater movement on a given site, so the system designer will typically take a conservative approach and assume minimal groundwater flow, which equates to potential temperature creep over time. Addressing this temperature creep requires that the designer consider the duration of his geothermal heat exchanger performance simulations. As the simulation period grows longer, the geothermal heat exchanger typically increases in size and cost. The challenge in the industry is to reduce the size and first cost of geothermal heat pump systems with minimal impacts on both energy efficiency and risk management regarding long-term geothermal heat exchanger performance.
DECOUPLED HYBRID GEOTHERMAL HEAT EXCHANGERS
An ideal hybrid geothermal heat exchanger might have the following characteristics:
• Ability to reject heat at optimal conditions of outside temperature and lowest electric rates;
• Ability to anticipate a future load and pre-condition the geothermal heat exchanger;
• Ability to thermally “reset” a geothermal heat exchanger on an annual basis if temperature creep occurs;
• Ability to optimize the geothermal heat exchanger temperature for peak overall system efficiency.
To achieve these characteristics, we need to consider several modifications to the traditional hybrid approach. The two modifications that are most significant include the ability to separate the geothermal heat exchanger flow from the building heat pump loop flow as well as applying intelligent control technology to track, compare, and anticipate thermal loads on the geothermal heat exchanger and its projected reaction to those loads.
Figures 4 and 5 illustrate a simplified configuration of this type of system when applied to either the central or distributed pumping concepts. The geothermal heat exchanger is hydraulically decoupled from the building heat pump loop, and dedicated pumps circulate fluid through the geothermal heat exchanger to control the building loop temperature. In an ideal situation where the building loads are balanced (cooling loads equal to the heating loads), these pumps could actually shut down. In reality, balanced loads are not that common, so a configuration of staged parallel pumping that allows the flow through the geothermal heat exchanger to drop to 10% to 15% of peak flow allows maximum pumping energy savings, as rarely will the overall system load drop below this minimum.
This configuration also allows the operation of the heat rejection device independently of building system flow. This allows potential diurnal and seasonal pre-conditioning of the geothermal heat exchanger when conditions are optimal for heat rejection and electrical rates are lower.
The essential component to make this work is an intelligent control system that has the sophistication to anticipate and prepare for future events. Prior to the advent of today’s advanced control technology platforms, this simply was not possible, and as a result, the above approach has been developed as the patent-pending intellectual property of Greensleeves LLC.
CASE STUDY
A new 225,000-sq-ft hospital in northern Mississippi is currently being designed with a geothermal heat pump system using distributed pumping as well as a decoupled hybrid configuration. This project has a site with limited area for the geothermal heat exchanger as well as specific project budget limitations. The initial full-size (non-hybrid) geothermal heat exchanger design exceeded both the available area and the project budget.
The traditional hybrid approach allowed a significant reduction in geothermal heat exchanger size (nearing 50%) but added significant fluid cooler energy consumption during the summer, allowed higher geothermal loop temperatures, which impact heat pump cooling efficiency and did not address a concern for temperature creep.
Applying a hybrid system comprised of a decoupled geothermal heat exchanger, a hybrid wet/dry fluid cooler, and an intelligent, self-learning control system allows a potential size reduction in the geothermal heat exchanger in excess of 60%. Also, the heat rejection can function nightly and seasonally under optimum conditions. Using a hybrid wet/dry fluid cooler also allows the owner to eliminate the chemical treatment and maintenance associated with a wet fluid cooler. The fluid cooler operates in the dry mode the majority of the time and only uses water if necessary.
While there is an energy penalty when a heat rejection system is added to a non-hybrid (full-size) geothermal heat exchanger, the amount of time required to recoup the additional first cost investment of a full-size geothermal heat exchanger on this project was well over 100 years. Due to the site and budget constraints on this project, the initial reaction was to move from geothermal to conventional HVAC technology, but with the decoupled hybrid approach, the project is back on track to leverage geothermal technology and deliver the optimal life-cycle performance the owner is seeking.
CONCLUSION
The two pumping schemes outlined herein each contain both positive and negative attributes. The centralized pumping system allows for centralized maintenance and higher wire-to-water efficiency at the pump, but it typically has a more complex control system and higher energy use, potentially two to five times higher than distributed pumping. The distributed pumping system has more pumps in remote locations, but has a simpler control system and has been shown to provide the lowest energy consumption when actual projects are reviewed.
As the wire-to-water efficiency of these smaller pumps continues to rise, it appears that much of the market will tend towards a distributed pumping solution to provide maximum energy performance. The application of a decoupled geothermal heat exchanger makes it easier to select a higher efficiency distributed pump due to the reduction in pump head needed at each of the distributed pumps. The decoupling also allows additional thermal management of the geothermal heat exchanger that would otherwise be inaccessible.
Advances in design and control technology allow us to continue to push forward in our quest for optimal HVAC system configurations. The availability of sophisticated simulation tools combined with smarter, self-learning control algorithms is allowing both significant reductions in geothermal system first cost while providing better energy efficiency and risk reduction.
The most significant market barrier is rarely the availability of advanced technologies or their cost, but instead a limitation of the number of practitioners that seek excellence in their designs. Marketplace options that pre-package this technology into a “plug and play” solution are making the application of optimized geothermal heat pump technology much more affordable and accessible. As these systems achieve widespread application and the performance feedback refines the control strategies, the market will see more reductions in first cost and improved energy efficiency causing the geothermal market to grow even faster than predicted. ES
CITED WORKS
1. Pike Research. “Research Report: Geothermal Heat Pumps and Direct Use.” Executive Summary. Q3 2011.
2. Kavanaugh, Stephen and Kevin Rafferty. “Ground-Source Heat Pumps: Design of Geothermal Systems for Commercial and Institutional Buildings.” Table 5.1, Benchmarks for GCHP System Pumping Efficiency. ASHRAE. 1997.
3. Kavanaugh, Stephen, Ph.D. “Less Pumping Means Cooler Ground Loops.” ASHRAE Journal. July 2011.
4. ibid.
5.. Hackel, Scott and Amanda Pertzborn. “Hybrid Ground-Source Heat Pump Installations: Experience, Improvements and Tools.” Energy Center of Wisconsin. June 30, 2011.
