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INTRODUCTION

The hybridization of geothermal heat pump systems (also known as ground-coupled or ground source heat pump systems) is accomplished by incorporating supplemental heat rejection or addition equipment, such as cooling towers, fluid coolers, boilers, and solar collectors, with the ground heat exchanger (GHE) loop. More generally, hybridization of geothermal heat pump systems could conceivably include the coupling of any heat source or sink to a GHE loop. Hybridization thus allows for part of the building thermal load to be exchanged via the supplemental equipment before heat transfer with the ground takes place.

In non-hybridized geothermal heat pump (GHP) systems that serve heavily heating- or cooling-dominated building thermal loads, an annual thermal imbalance of the ground thermal loads will occur. For instance, in heating-dominated buildings a non-hybridized geothermal heat pump system will on an annual basis extract more energy from the ground than reject to it, causing the average temperature of the ground volume to decrease over time. As the average ground temperature decreases, the thermal quality of the heat source for the heat pump cycle is degraded (a heat source at a progressively lower temperature), causing the coefficient of performance (COP) of the heat pump to deteriorate. Similarly, in coolingdominated buildings more energy is rejected to the ground than extracted from it, and on an annual basis the average ground temperature will increase, resulting then in a thermally degraded heat sink (a heat sink at a progressively higher temperature) for the heat pump cycle. Thermal imbalance conditions in the ground will cause the GHP system to operate at increasingly reduced capacities, and may ultimately result in system failures due to continuously deteriorating heat pump COP. In order to avoid failure without hybridization in heating- or cooling-dominated buildings, ground heat exchanger loops must be sized to satisfy annual peak heating and cooling loads for the entire life span of the system, which requires excessively large and costly ground heat exchanger loops and borehole fields.

The significance of hybridization lies in the fact that it may be used to completely balance ground thermal loads on an annual basis, thus not allowing sink/source thermal degradation of the heat pump cycle to occur. Furthermore, balancing ground loads annually by shifting the unbalanced portion to a supplemental heat transfer unit removes an implicitly built-in limitation in life span of energy-efficient operation in such systems. A thermal balance in the loading of the ground volume via ground loop heat exchangers is achieved so that on an annual basis the magnitude of energy extracted equals the magnitude of energy rejected, ensuring maximum and minimum heat pump entering fluid temperatures (temperature of the heat transfer fluid returning to the heat pump from the ground) to remain constant within an acceptable range for the operation of the heat pump cycle at designed efficiencies. Thermal balancing of the ground loads implicitly sizes the ground loop heat exchanger loop for the less dominant building load at the allowable heat pump entering fluid temperatures, and as a consequence hybrid systems permit the use of smaller, lower-cost borehole fields. However, the design of hybrid systems adds to the complexity of the overall GHP design process because of the addition of another transient component to the system. For example, as building loads display a time-dependent behavior, acceptable conditions for supplemental heat rejection to the atmosphere in a coolingdominated building and for solar recharging of the ground in a heating-dominated building are also time-dependent functions of weather conditions, solar availability and ground loop temperature. Consequently, hybrid GHP systems are best analyzed on an hourly basis (as typical weather data are also available in hourly time-steps) for the accurate and reliable assessment of the overall system thermal behavior.

The accurate design of hybrid GHP systems is essentially an optimization problem as sizing of the supplemental components (required solar collector area, cooling tower or fluid cooler capacities) and the GHE loop length stipulate the management of multiple degrees of freedom on multiple system design parameters under constraint conditions of annual thermal load balance in the ground at a desired entering heat pump fluid temperature range. In addition, the design of hybrid GHP systems must use an appropriate control algorithm for system operation for load balancing in the ground. Clearly, proper, accurate and reliable design of hybrid GHP systems is quite difficult and cumbersome without the use of a detailed system simulation approach. Furthermore, without an automated optimization scheme coupled to the system simulation program, the design activity itself can become tediously impractical and time-consuming.

The overall goal of the work presented in this paper is to develop an optimization approach for the design of hybrid GHP systems in heating- and cooling-dominated buildings that effectively balances ground thermal loads, based on current "state-of-the-art" system simulation methods. It should be noted however that balancing ground loads may or may not yield an economically optimized system as the approach is based on minimizing ground loop length under the assumption that the ground loop is the most costly portion of the system. One could choose to optimize a system based on life-cycle cost using a similar system simulation approach. However, attempting to find an optimum life-cycle cost is fraught with uncertainties in economic indicators and future energy prices. For this reason, a design approach of minimizing total ground loop length by balancing annual ground loads is preferred, and economic calculations may still be made using the sizing results of the optimization approach proposed in this paper.