Low Enthalpy Geothermal Open Loop Heat Pumps: a suitable tool for thermal energy supply in urban areas.

Geothermal heat pumps represent an interesting technology that is expected to contribute significantly to the reduction of primary energy use for heating and cooling and meet the targets set by the European Union. Additional benefits of this technology are related to the integration with discontinuous energy resources, in particular wind, combining heat and power. The replacement of conventional heating systems such as boilers, with heat pump systems allows the de-localization of emissions of micropollutants from urban centers to the sites in which thermal power stations are operating. This also enhances emissions monitoring and control. Furthermore, the use of distributed production systems based on the use of renewable sources reduces also CO2 emissions. (Lo Russo et al., 2011) In this general context, the increasing implementation in several areas of the world of the open-loop groundwater heat pumps technology which discharge into the aquifer for cooling and heating buildings could potentially cause, even in the short term, a significant environmental impact associated with thermal interference with groundwater, particularly in the shallow aquifers. The discharge of water at different temperatures compared to baseline (warmer in summer and colder in winter) poses a number of problems in relation to the potential functionality of many existing situations of use of the groundwater (drinking water wells, agricultural, industrial, etc.). In addition, there may be cases of interference between systems, especially in the more densely urbanized. This means that the alteration of the temperature of the groundwater determined by a plant may affect the installations located downstream, with significant alterations of the performances of the systems themselves. These issues highlight how it is crucial for the compatible development of the technology of groundwater heat pumps discharging into aquifers that it shall be a fair assessment and technically effective both for cooling and heating plants and pumping and injection systems in ground. The current legislation related to withdrawals and discharges into aquifers design a framework suitable for the protection of groundwater and permit to decide the 3 best configuration of the plant with a case by case approach. Appropriate specialized hydrogeological investigations should be performed for the characterization of the main hydrogeological parameters of the subsoil at the considered site. In this thesis some important aspects related to the development of open-loop heat pumps have been explored in a typical urban contest (Torino city, NW Italy). The results of the work have allowed to define several fundamental aspects in order to optimize the design choices of GroundWater Heat Pump (GWHP) systems. After a general description of the low enthalpy geothermal heat pumps technologies (Chapter 1), the analysis and comparison of the current hydrogeology problems in urban area are described, considering the impact of groundwater heat pump system in a urban contest (Chapter 2). Urban and industrial development can impose major stresses on groundwater resources. The conceptual model for the groundwater flow system, the schematization of the aquifer boundaries and the estimation of basic hydrogeological parameters are among the main issues which should be investigated in the development of open-loop heat pumps plants. In particular, some characteristics of urban elements require particular attention if compared to less anthropized areas. In Chapter 3 the geological and hydro-stratigraphical characteristics of the Torino test site have been described. This chapter includes a complete description of the GWHP system plant and monitoring system that has been installed in the Politecnico di Torino and the illustration of the fundamentals of the numerical modelling we performed using a specific commercial code (FEFLOW® Diersch, 2010). In Chapter 4 the relative significance of the different subsurface parameters that mostly characterize the developed thermal plume is determined through a detailed sensitivity analysis under different simulation conditions. In Chapter 5 we explore the importance of the compliance between real and simulated variable input flow data (discharge and injection temperatures) to obtain reliable simulation results. In the following Chapter (6) we explore the potential alternative to Finite Element Modeling (FEM) tools in the spatial and temporal prediction of the thermal plume development by considering the use of Artificial Neural Networks (ANNs). Finally, in 4 Chapter 7, we consider the potential alternatives to traditional vertical drilled wells to disperse the thermal energy in the aquifer comparing such technology with the alternative use of gabions draining. The results highlighted some important aspects that should be considered in the modeling of the open-loop heat pumps that are summarized in the Conclusions. The research individuated some important aspects and reached important results but, as clearly highlighted, several aspects of the analysis of these kind of technology should be further investigated by research and practical monitoring observations in the future. However, even taking into account all the limitations of the open-loop heat pumps technology, we believe that these systems represent one of the most promising potential clean energy source especially in the urban areas under transformation in order to reach the important goal of greenhouse-gas emission reduction of the future Smart liveable Cities.