Hybrid Ground-Source Heat Pump Installations: Experiences, Improvements, and Tools

One innovation to ground-source heat pump (GSHP, or GHP) systems is the hybrid GSHP (HyGSHP) system, which can dramatically decrease the first cost of GSHP systems by using conventional technology (such as a cooling tower or a boiler) to meet a portion of the peak heating or cooling load. This work uses three case studies (two cooling-dominated, one heating-dominated) to demonstrate the performance of the hybrid approach. Three buildings were studied for a year; the measured data was used to validate models of each system. The models were used to analyze further improvements to the hybrid approach, and establish that this approach has positive impacts, both economically and environmentally. Lessons learned by those who design and operate the systems are also documented, including discussions of equipment sizing, pump operation, and cooling tower control. Finally, the measured data sets and models that were created during this work are described; these materials have been made freely available for further study of hybrid systems

Extremum Seeking Control of Hybrid Ground Source Heat Pump System

The ground source heat pump (GSHP) technology is a renewable alternative for space conditioning by rejecting/absorbing heat to/from the ground, which has demonstrated higher energy efficiency for residential and commercial buildings. As the system capacity is limited by the initial cost of construction of ground-loop heat exchanger (GHE), developing the so-called Hybrid GSHP system by utilizing supplemental heat rejecters such as cooling towers has emerged as a cost-effective alternative. In practice, operational efficiency of Hybrid GSHP system mainly depends on 1) the actual characteristics of heat pump, cooling tower, GHE and other equipment; 2) ambient air and ground conditions. In particular, the GHE heat transfer is heavily affected by the ground thermal characteristics which, however, is difficult and expensive in practice to determine due to the complexity of soil type and distribution. In addition, the actual cooling tower characteristics can vary significantly. Such uncertainties bring forth dramatic difficulty for successful application of model based control or optimization methods. In this study, an extremum seeking control (ESC) strategy is proposed for efficient operation of a hybrid GSHP system with cooling tower, which minimizes the total power (i.e. GHE loop water pump, cooling tower fan and pump, and the heat-pump compressor) consumption by tuning the air-flow rate of the cooling tower fan and the GHE loop water flow rate. To evaluate the proposed control method, a Modelica based model of the Hybrid GSHP system is developed by utilizing the Buildings Library developed by the Lawrence Berkeley National Laboratory, which consists of a 20-borehole GHE, a water-to-water heat pump, a counter-flow cooling tower and a plate heat exchanger. The transient conduction model of vertical GHE in the Buildings Library is adopted, which is based on a finite-volume method inside the borehole and cylindrical source model outside the borehole. A variable-flow water pump model is constructed for the GHE water loop, which gives power consumption under different operating scenarios. A cooling tower model in the Buildings Library is adopted, which is a static polynomial model based on a York cooling tower correlation. The relative air flow rate can be regulated to maintain the leaving water temperature at the setpoint, and then the corresponding fan power consumption is obtained. The heat pump model is based on the evaporator temperature, condenser temperature and Carnot efficiency. An inner-loop proportional-integral (PI) controller is implemented to regulate the evaporator leaving water temperature at 7 deg-C. Under the air wet-bulb temperature of 35 deg-C and dry-bulb temperature 23 deg-C, steady-state simulation of the plant model yields the static map of the total power with respect to the cooling tower relative air flow rate and the GHE water flow rate, which indicates about 25% power variation across the adjustable range of inputs. Simulation was conducted in two conditions: change in evaporator inlet water temperature and change in ambient air condition. The simulation study under way is to validate the effectiveness of the proposed ESC strategy, and the potential for energy saving will also be evaluated

An Examination of Control Strategies in a HyGSHP System

Hybrid ground source heat pump (HyGSHP) systems are gaining popularity as a means of decreasing long term energy and maintenance costs while maintaining manageable first costs. The incorporation of a supplemental heat rejecter into a standard GSHP design introduces several complexities. One of the major challenges involves determining the optimal control strategy for such a system. This paper includes a review of HyGSHP control strategies followed by a more detailed discussion of the pre-cooling strategy. This strategy incorporates thermal storage into the system in order to shift power consumption to periods with off-peak electric rates, thereby reducing operating costs. The results of a preliminary study of pre-cooling as well as plans for further research will be presented

Modeling, Verification and Optimization of Hybrid Ground Source Heat Pump Systems in Energyplus

Hybrid Ground Source Heat Pump system simulation capability was added to EnergyPlus. Program was modified to handle multi-year simulation capability. New supervisory controls were added and a plate heat exchanger model developed to configure realistic systems. The models were verified by comparing to the published results in literature by Yavuzturk et al. (2000). GenOpt was used to investigate the optimal design, control strategies and configuration. Life cycle cost of the system was used as objective function. Effects of using first year operating cost and 20th year operating cost on optimization results were studied. The modifications and addition of new models successfully modeled HGSHP systems in EnergyPlus. The results obtained from EnergyPlus model agreed well within the modeling differences with Yavuzturk results. Optimization proceeded to decrease the GLHE length to the minimum possible, since it was the highest contributor towards cost. Optimization of HGSHP systems was found to be system specific and highly affected by the load imbalance. Use of 20th year operating cost instead of first year operating cost to calculate the life cycle cost affected the results of optimization.Mechanical & Aerospace Engineerin

Optimal Design for a Hybrid Ground-Source Heat Pump

Although the advantages of ground-source heat pumps over their conventional alternatives make these systems a very attractive choice for air conditioning, not only for residential buildings but increasingly also for institutional and commercial buildings, a significant barrier to wider application of this technology is a high first cost. When used in cooling-dominated buildings, ground-source heat pumps that utilize vertical, closed-loop ground heat exchangers can experience performance degradation as the entering fluid temperature to the heat pump increases over time due to heat buildup in the borefield. In these cases, it is possible to displace a large portion of the system cost by installing a supplemental heat rejecter to balance the annual heat extraction from the ground. The paper presented has shown that the heat rejection of the GLHEs and the system energy consumption are approached to discuss the ground heat balance with different design procedures and control strategies though the system simulation

Simulation and Optimal Control of Hybrid Ground Source Heat Pump Systems

The objective of the study was development of optimal control strategies for hybrid ground-source heat pump (HGSHP) systems in order to improve the performance of HGSHP systems. The investigations of the four sub-objectives were carried out, including: development of HGSHP system simulation and requisite component models, validation of HGSHP system and component simulation, development of an HGSHP system design procedure, investigation and development of generally applicable optimal control strategies.All requisite component models for the HGSHP system simulation have been developed. The HGSHP system simulation has been implemented in HVACSIM+. The HGSHP system simulation was validated against the experimental data collected from the OSU HGSHP research facility. After the calibration of each component model, the simulation gave total energy consumption about 0.2% higher than the experiment.A parallel-connected HGSHP system was chosen for the study and a strategy for controlling the flow distribution between GLHE and PHE/cooling tower was developed. A temperature limit-optimized simulation-based method was developed for sizing the HGSHP system components and was implemented in GLHEPRO. A comparative study of three hybrid ground source heat pump system design procedures was carried out. In general, the HGSHP designed from the new design procedure would have a smaller GLHE and cooling tower size and a smaller system life cycle cost than the system designed from the Kavanaugh and Rafferty procedure (1997).Two building types and six U.S. cities were chosen to provide different building load profile for the study of HGSHP system controls. Three control strategies developed by Yavuzturk and Spitler (2000) were investigated first. The investigation results showed, for the three control strategies, there is no generally-applicable setpoint for different combinations of HGSHP system design, building type and location. Three new control strategies have been developed in this research: a system load control strategy, a forecast/historical control strategy and a varied EFT/ExFT control strategy. Using these new control strategies, without individually optimizing setpoint for a specific building and location combination, the systems will have savings from 1% to 26% compared to the base cases.Mechanical & Aerospace Engineerin

Modeling of vertical ground loop heat exchangers for ground source heat pump systems

The ability to predict both the long-term and short-term behavior of ground loop heat exchangers is critical to the design and energy analysis of ground source heat pump systems. For detailed analysis and accurate simulation of the transient heat transfer in vertical ground loop heat exchangers, a numerical model is developed. The model is based on the two-dimensional, fully implicit finite volume formulation and utilizes an automated parametric grid generation algorithm for different pipe sizes, shank spacing and borehole geometry. The numerical method and grid generation techniques have been validated against an analytical model. The numerical model has been developed with two main purposes in mind. The first application is the calculation of non-dimensional temperature response factors for short time scales that can be used in building simulation. The second application is use in a parameter estimation technique used to predict borehole ground formation thermal properties from short time scale test data.The short-term behavior of ground-coupled heat pump systems is important for the design of ground loop heat exchangers, the energy analysis of ground source heat pump systems, and the design of hybrid ground source systems. Using short time-step response factors, a direct evaluation of system energy consumption and electrical demand in hourly or shorter time intervals becomes possible since a detailed assessment of the ground heat exchanger behavior on an hour-by-hour basis can be performed. This is important especially when dealing with strong short time-step system fluctuations due to building dynamics and for commercial buildings that have time-of-day electricity rates. The short time-step model is cast as a TRNSYS component model and validated using actual operating field data from an elementary school building located in Lincoln, Nebraska.Furthermore, a short time-step ground loop heat exchanger model is crucial for analysis of hybrid ground source heat pump systems. Ground source heat pumps for cooling-dominated commercial buildings utilize supplemental heat rejecters such as cooling towers, fluid coolers or surface heat rejecters to reduce system first cost and to improve system performance. The use of supplemental heat rejecters for cooling dominated buildings allows the design of smaller borehole fields. Heat pump performance degradation is avoided by offsetting the annual load imbalance in the borefield and the resulting long-term temperature rise. Utilizing the short time-step model, a parametric study is presented to investigate the advantages and the disadvantages of various system operating and control strategies in a hybrid ground source heat pump application under different climate conditions. An actual office building located in Stillwater, Oklahoma is used as the example building. A preliminary life cycle cost analysis is conducted to compare each operating and control strategy to determine the lowest cost alternative for a given climate.The numerical model is also used as part of parameter estimation algorithm that is developed to predict borehole ground formation thermal properties from short time scale test data. Determination of the ground's thermal conductivity is a significant challenge facing designers of Ground Source Heat Pump (GSHP) systems applied in commercial buildings. The number of boreholes and the depth and cost of each borehole are highly dependent on the ground thermal prope11ies. Hence, depending on the geographic location and the local drilling costs, the ground thermal properties strongly influence the initial cost to install a GSHP system. In order to be able to predict ground thermal properties, a parameter estimation technique is employed that minimizes the sum of the squares error between experimentally measured temperature responses and the temperature predictions of the numerical model. An experimental apparatus has been built capable of imposing a heat flux on a test borehole, and measuring its temperature response. The downhill simplex method of Nelder and Mead (1969) in conjunction with the two-dimensional numerical model is used to determine the thermal conductivity of the surrounding ground. In order to validate the procedure, independent measurements of the soil conductivity test results are reported for several test boreholes and a laboratory experiment. A detailed uncertainty analysis of the thermal conductivity prediction is conducted to assess the impact of uncertainty of a series of input parameters

Simulation Study of Hybrid Ground Source Heat Pump System in the Hot-Humid Climate

The beachfront hotel with hybrid geothermal heat pump system (HyGSHP), located in the hot-humid climate, is simulated by TRNSYS in the thesis, and the simulation results are validated by the measured data. The simulation of alternative HVAC systems, complete ground source heat pump and conventional air source heat pump, are included to conduct the comparative study with HyGSHP based on the energy consumption and life cycle analysis. The advantages and disadvantages of HyGSHP are discussed in the thesis. Two ground source heat exchanger parameters, U-tube size and grout materials, are investigated in order to study the effects on the ground heat exchanger thermal performance. The preliminary work and results are shown in the thesis

Dynamic modeling and control of hybrid ground source heat pump systems

Ground source heat pump (GSHP) systems are one of the fastest growing applications of renewable energy in the world with annual increases of 10% over the past decade. GSHPs are potentially more efficient than conventional air-to-air heat pumps as they use the relatively constant temperature of the geothermal energy to provide heating or cooling to conditioned rooms at desired temperature and relative humidity. More importantly, GSHP systems can in fact achieve significant energy savings year round, compared to conventional HVAC systems. A hybrid ground source heat pump (HGSHP) system is designed in this study to heat and cool an office building all the year round. Dynamic models of each component of the heat pump system are developed for simulations of heat transfer between each component of the HGSHP system and for control strategy design and analysis. A detailed multiple-load aggregation algorithm (MLAA) is adapted from the literature to precisely account for and calculate the transient heat conduction in vertical ground heat exchangers with different yearly, monthly, and daily pulses of heat. Feedback PI controllers for heat pump units and On/Off controllers for boiler and cooling tower are designed and utilized to match anticipated building loads and to analyze transient response characteristics of such outputs as water flow rate and air flow rate of heat pumps, return water temperature and supply air temperature of heat pumps, water temperatures of ground loops and heat exchangers, water temperature of boiler or cooling tower, and fuel flow rate of boiler. Control strategies for the HGSHP system in both heating and cooling modes of operation are also introduced to study the system responses. With the usage of On/Off controllers and well-tuned PI controllers, as well as optimal control strategies for heating and cooling operations, the HGSHP system is expected to give better operating performance and efficiency. As a result, noticeable energy savings can be achieved in both heating and cooling modes of operatio

Imulation and Validation of Hybrid Ground Source and Water-loop Heat Pump Systems

This study focused first on the simulation and validation of hybrid ground source heat pump (HGSHP) systems. Validation of such systems is previously unreported in literature. The simulation was done using HVACSim+. Validation was done using seven months (March to September 2005) of five-minutely experimental data from an HGSHP research facility located on the campus of Oklahoma State University (OSU). The validation results were considered from the perspective of both researchers and designers with regards to accuracy of HVAC energy consumption prediction. The second part of this study focused on the simulation, validation, and control optimization of water-loop heat pump (WLHP) systems. The WLHP simulation was also done using HVACSim+. Four days (September 22-25, 2006) of five-minutely data from the OSU HGSHP research facility were used for experimental validation purposes. Intermodel validation was done between HVACSim+ and EnergyPlus to further validate the model. Control optimization was done on two building types in 13 U.S. cities. Single setpoint controls, dynamic controls, controls based on outdoor wet-bulb temperature, and controls utilizing forecasting with thermal mass augmentation were considered.Mechanical & Aerospace Engineerin