Simulation Model The simulation model described here has been presented, in considerable detail, by Yavuzturk and Spitler A brief description will be presented in this paper. The model is primarily aimed at appli- cations in building energy analysis, where it is desirable to be able to predict system energy consumption on an hourly basis. For short duration heat pulses, heat transfer within the borehole and heat transfer outside the borehole, in the radial direction, are much more important than heat transfer in the axial direction.
Hence, a two-dimensional, radial-angular, finite volume model has been developed. Complete details may be found in Yavuzturk, et al. The numerical model is used to calculate the average fluid temperature in the borehole.. This is then adjusted by the borehole resistance to determine the average temperature at the borehole wall and then non-dimensionalized to form a g-function. The resulting short time-step g-function curve matches well at the boundary to the long time-step g-functions developed by Eskilson, as shown in Figure 3.
Loads that occurred more recently are treated as hourly pulses. This approach gives significantly reduced computational time, while maintaining very good accuracy. The load aggregation procedure is given in more detail in the paper by Yavuzturk and Spitler Experimental validation of the model is described in detail by Yavuzturk and Spitler Standing Column Well Standing column wells are used for direct i. Figure 4 il- lustrates the configuration of a standing column well and the heat and mass transfer mechanisms.
A nu- merical model has been developed for studying the performance of the standing column wells, which is composed of two parts: a nodal model of the borehole components and a finite volume model of the ground-water flow and heat transfer in the surrounding rock. This model allows the explicit treatment of the advective heat transfer induced by the ground-water flow Rees, et al.
More recent work Deng has focused on development of a simpler, much faster model, suitable for use in energy analysis programs or design programs. Figure 4: Standing column well 2. Water Source Heat Pump Jin and Spitler developed a parameter-estimation-based water-to-water heat pump model. This model used a thermodynamic analysis of the refrigeration cycle, simplified heat exchanger models, and a detailed model of the refrigerant reciprocating compressor.
Jin described in detail the multi-variable optimization and the estimated parameters. Jin also presented an analogous model for water-to-air heat pumps. Shenoy developed a simpler but faster equation-fit type model for water-to-air heat pumps.
Supplemental Heat Rejecters for Hybrid GSHP Systems The cost of drilling the borehole field for a ground-source heat pump system, although it depends strongly on the local geological conditions, can often be a substantial portion of the system capital cost.
This is most likely in buildings where the demand is predominantly for cooling. In situations like this and where the thermal conductivity of the ground is low or drilling conditions are poor, the cost of the bore- hole field may make a ground-source system uneconomical.
However, a compromise between first cost and energy efficiency may be possible by using a smaller borehole field and adding a supplemental heat rejecter into the heat pump water loop. A number of different types of heat rejecters have been suggested for inclusion in the water loop of hybrid systems, including cooling tower, shallow pond with heat exchanger, and hydronically heated pavement or bridge deck.
The shallow pond model has been developed by Chiasson, et al. Chiasson, et al. This model has been further developed to be able to model the snow melting process taking place on the top surface of the slab Liu, et, al.
These models have been validated using data from a number of experimental supplemental heat rejecters at Oklahoma State University.
In such cases the ground temperature may rise or fall over a number of years, resulting in a decrease in the performance of the heat pump as the entering fluid tem- perature to the heat pump rises or falls. A design goal must therefore be to control the change in the temperature within acceptable limits over the life of the system. The net heating or cooling of the ground over each season clearly depends on the accumulated heat rejection and extraction, and therefore on the building loads throughout the whole year.
The design meth- odology has to be based then on the building loads calculated throughout the whole year, not just the peak heating and cooling loads. Hence more information is required regarding the building loads than for siz- ing of a conventional system.
Design methodologies available for residential ground loop heat exchangers have been reviewed by Cane and Forgas Yavuzturk provides a more up-to-date review of all available methodolo- gies.
Kavanaugh described a design procedure commonly used in the United States. A design methodology that utilizes the simulation procedures described in Section 2 is covered in de- tail by Spitler However, the peak heat extraction or rejection pulse has been modeled with a simpler analytical approximation.
The design methodology has been implemented in a commercially available software package Spit- ler , Assuming a given borehole depth, and the above information, the average fluid temperature in the GLHE at the end of each month, the entering fluid temperature at the end of each month, and the actual heat rejection rate for each month are determined simultaneously. Then, the responses to the peak pulses are determined for each month, and the resulting peak entering fluid temperatures to the heat pump s for each month are determined.
The program also has a sizing mode where the minimum borehole depth that will meet user-specified minimum and maximum peak temperatures is determined by searching with the simulation until the depth is found that is constrained by either the minimum or maximum peak entering fluid temperature. Hourly or shorter time steps may be used so that diurnal variations in loop temperature, time-of-day rates, etc. Ultimately, it is anticipated that this approach will be used to design GSHP systems.
To date, it has been primarily used for feasibil- ity studies and parametric investigations. Three types of sample applications are discussed below. With the supplemental heat rejecter s , the size of the ground loop heat exchanger may be reduced significantly. Perhaps the most obvious candidate for a supplemental heat rejecter in a hybrid system would be a conventional open-circuit cooling tower.
Yavuzturk and Spitler investigated a number of operat- ing strategies for an HGSHP system with a cooling tower, using the simulation methodology described above. One method investigated for controlling the cooling tower was to switch on the cooling tower only when a certain heat pump entering water temperature was exceeded. It was found that this simple strategy does not result in the cooling tower operating during the most advantageous weather conditions.
A second strategy studied involved operating the cooling tower on a predetermined schedule. The most effective strategy was found to be one where the tower was controlled by the difference between the heat pump entering fluid temperature and the wet bulb temperature. This allows the cooling tower to be used under the most advantageous weather conditions where the potential is greatest for heat rejection.
For the climate conditions that were considered, this control strategy yields the lowest life cy- cle cost. Another possible supplemental heat rejecter is a pavement heat exchanger, consisting of pipes buried just below a road or other paved surface. In a recent study Khan et al. For the particular case examined here, the addition of a 36 m2 heated parking lot allowed a reduction in size of the GLHE from 16 to 9 boreholes, each 73 m deep.
With the addition of the parking lot heating system, operating costs decreased slightly and the system also per- formed some snow melting, reducing the number of hours that the parking lot would be snow-covered without other intervention from to Shallow ponds have also been suggested as possible heat rejecters for use in hybrid ground-source heat pump systems.
Where ponds are required for either landscaping, irrigation or flood control purposes it is relatively simple and cost effective to introduce additional pipe coils at the bottom of the pond con- nected into the loop with the borehole field.
A parametric study Ramamoorthy et al. Hybrid GSHP systems that use solar thermal collectors for diurnal and seasonal thermal underground energy storage have been studied Chiasson and Yavuzturk for applications in cold climates. The study models an actual school building with typical meteorological year weather data for a number of cit- ies with varying climates and insolation. The results of the study have shown that hybrid solar GSHP sys- tems are a viable choice for space conditioning of heating dominated buildings.
Building cooling may be provided by circulating water directly between the ground loop heat exchanger and chilled ceilings or beams. In this system, the heat extracted from the ground for heating is replaced during the cooling season, and cooling is provided at almost no cost. Spitler and Underwood pre- sented a simulation study of this application for a five-story office building in Newcastle.
Such a system can potentially offer improved road safety and increased bridge deck life. A number of heat sources have been proposed for such systems, including heat pipes, natural gas boilers and electric cables.
GSHP systems and hydronic-heating offer improved energy efficiency over other systems. Such systems consist of hydronic tubing embedded in the bridge deck with hot water circulated from a number of water-to-water heat pumps that, in turn, extract heat through the ground via vertical U-tube borehole heat exchangers, as shown in Figure 6. This geo- thermal bridge deck technology has been the subject of a recent research project at Oklahoma State Uni- versity. Application of geothermal bridge deck snow melting technology has been discouraged by higher ini- tial costs, but also lack of reliable design procedures, modeling methods and software tools.
The simulation results have been validated with operating data and corresponding weather data from a medium scale experimental bridge snow melting system that employed a GSHP system as heat source.
The validation results show the ability of this simulation approach to successfully predict the performance of the system under a wide range of operating and weather conditions. Since the design of a ground source heat pump system has many degrees of freedom, and the interaction between the design variables is relatively com- plex, automated optimization is a potentially useful tool for GSHP system design. Some preliminary work in this area has been presented by Khan and Spitler and Khan The first reference presents a study of residential systems, where a few variables are optimized at any one time.
The detailed system simulation accounts for the effects of antifreeze concentration, entering fluid temperature and flow rate on heat pump capacity. Mass flow rate in the system was solved for every time step and varied with the fluid transport properties, which depended on temperature and antifreeze concentration.
Life cycle cost was chosen as the objective function. Optimizing a few variables at a time allowed the determination, for ex- ample, of optimal GLHE size and antifreeze concentration, if all other variables were held constant.
Khan reported on an attempt to simultaneously optimize all variables using both a particle swarm optimization algorithm and the Hooke-Jeeves algorithm. Ultimately, these attempts met with lim- ited success — in order to be confident that the constraints on freezing and unmet loads were really and truly met, it was necessary to perform a multi-year e.
Future work with larger time steps in the early years may facilitate optimization, but no conclu- sion has yet been reached as to the potential for reductions in life cycle cost due to optimal design. Models of the main components of the GSHP system are now available. The long-term and short- term performance of the ground-loop heat exchanger can be predicted.
The performance of the standing column well can be predicted with a numerical model. In addition, models of water source heat pump and different forms of supplemental heat rejecter have also been developed.
By utilizing these component models in a modular simulation environment, a number of previously impractical simulations have been performed. These simulations have been applied in the design a variety of hybrid ground source heat pump systems.
Parametric studies performed with these simulations have led to new insights about system design. A procedure for the design of vertical closed-loop ground heat exchangers has been developed from the earlier work of Eskilson. The procedure takes account of the cumulative effect of the building loads rejected to and extracted from the ground loop on its long-term performance.
With this procedure, the required length of the boreholes can be calculated to maintain the heat pump entering fluid temperature within its design limits over the life of the project.
Preliminary attempts at automated optimization have yielded only modest successes. Additional work in the near future should, at least, indicate more clearly what potential savings in life cycle cost might be achieved with optimal design procedures.
Modeling of ground-source heat pump performance. Cane, R. D and Clemes, S. Morrison and C. Chiasson, A. Spitler, S. Rees, M. Surface water heat pumps and ground water heat pumps are covered, and special focus is given to both vertical and horizontal ground-coupled heat pump systems, for which modelling and simulation is discussed, and experimental systems are described.
Due to its advanced approach to the subject, this book will be especially valuable for researchers, graduate students and academics, and as reference for engineers and specialists in the varied domains of building services. Explores fundamentals and state-of-the-art research, including ground-coupled heat pump GCHP systems. Includes performance assessment and comparison for different types of GSHP, numerical simulation models, practical applications of GSHPs with details on the renewable energy integration, information on refrigerants, and economic analysis.
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