Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 Proposal of Technical Guidelines for Optimal Design of Ground-Source Heat Pump Systems Paolo Conti*, Walter Grassi and Daniele Testi DESTEC (Department of Energy, Systems, Territory and Constructions Engineering), University of Pisa, Largo Lucio Lazzarino, 56122 – Pisa (IT), UGI (Italian Geothermal Union) members *Corresponding author: paolo.conti@for.unipi.it Keywords: Ground-Source Heat Pumps, Design Guidelines, Holistic Design, Integrated Design, Seasonal Performance, Optimization, Ground-Coupled Heat Exchangers, Cost-Benefit Analysis. ABSTRACT An innovative design methodology for ground-source heat pump (GSHP) systems has been developed, based on the evaluation of energy exchange and performance during the entire operational life. This novel procedure takes into account design solutions in which GSHPs are coupled with other heating and cooling technologies and finds the reciprocal optimal shares of thermal loads in terms of cost-benefit indicators. The proposed method is holistic; in other words, it incorporates in a single set of equations all the interactions among the three macro-systems governing the energy balance of GSHPs: building thermal energy loads, efficiencies of generators (heat pump and back-up systems), and thermal response of the ground (taking into account the sustainability of the source). The optimal design parameters and energetic and economic outputs of the procedure are: - thermal capacities of the heat pump and back-up generators; - size, number and position of ground heat exchangers; - flow rate in the ground-coupled loop; - load shares between GSHP and back-up systems (control strategy); - required energy input during multi-year operation; - energy savings with respect to the exclusive use of conventional back-up systems or, conversely, to the use of the sole geothermal system; - installation and operational costs; - key investment indicators. Guidelines to be followed by professionals for an effective design procedure in the case of ground-coupled vertical borehole heat exchangers (BHEs) are illustrated step-by step. 1. INTRODUCTION Heat pump systems are a widely used technology for thermal energy generation, capable of efficiently delivering heating, cooling, and sanitary hot water for buildings. Particularly, ground-source heat pumps (GSHPs) are potentially able to reach higher performances with respect to their traditional alternatives (e.g. air heat pumps, condensing boilers, solar technologies), provided that special attention is paid to the initial design of the overall system (heat pump equipment, ground heat exchanger, and connecting ductwork). The installation design must be the product of the complete view of the building needs, the system for energy production, the distribution system and controls, and the characteristics of the ground source. Several standards are already available to designers (ASHRAE, 2011; Kavanaugh and Rafferty, 1997; UNI, 2012a; VDI, 2001) however, most of these methods are based on some operative parameters decided a priori (e.g. ground-coupled loop temperatures and flow rates, GHP capacity, heating/cooling load share assumed by the ground source, reference design month). As a consequence, despite their practical usefulness, current design procedures do not guarantee that the final design is the most cost- effective in terms of operative performance. An alternative approach to GSHP design has been proposed in scientific literature (Retkowski and Thöming, 2014; Robert and Gosselin, 2013). In these works, borehole heat exchangers (BHEs) number, depth and spacing, together with GHP unit capacity, are optimized according to their impact on final operative performances. Additionally, Conti et. al (2013, 2014a) include the management strategy of the system within the design variables, showing the energetic and economic benefits that can be reached through an appropriate synergy of geothermal source and back-up generators. In the following sections, we describe a general design method based on the optimization of the operative performance of GSHPs with vertical BHEs, integrating it in a straightforward procedure. This method can be applied by the professionals to identify the optimal design solution among possible equipment alternatives. Finally, a test case is presented in order to compare the results of traditional methodologies with the present approach. 2. PRELIMINARY CONSIDERATIONS ON GSHP OPTIMAL DESIGN The performance of a GSHP system can be evaluated in terms of both energetic and economic savings with respect to other technologies. Optimal design is obviously dependent on the selected performance index (e.g. primary energy consumption or total costs), specific technical constraints, and economic context of the project; however, some general elements can be identified to set up the design procedure. First of all, it is worth recalling that ground-coupled systems usually work in synergy with other generation technologies. The use of a GSHP system is convenient only when operative conditions (source temperatures and unit capacity ratio) allow delivering useful thermal energy with an energy consumption lower than back-up technologies. In other words, a proper design has to include, among other variables, the optimal shares of the building thermal load among the different generation systems. In the present work, we refer to capacity ratio (CR) according to the following definition: