Sensitivity Analysis on the Performance of a Ground Source Heat Pump Equipped with a Double U-pipe Borehole Heat Exchanger

AbstractGround Source Heat Pumps (GSHP) are economically and environmentally advantageous for the heating and cooling of buildings, provided that the long-term sustainability of the thermal exploitation of the soil is ensured. In particular, the performance of a closed-loop Borehole Heat Exchanger (BHE) strongly depends on the geometrical and physical properties of its components and on the thermo-hydrogeological properties of the surrounding soil. In this work, we present the results of a series of simulations of a double U-pipe Borehole Heat Exchanger, carried out with the finite-element flow and heat transport modelling software FEFLOW to assess the relative influence of these parameters on the operation of a GSHP. The analysis confirms that the length of the borehole is the main design parameter, but the thermal conductivity of the grout, the pipe spacing, the heat carrier fluid and its flow rate also have an important effect on the energy efficiency of the system. The thermal conductivity of the soil is another fundamental variable in the design of a GSHP, and hence it is better to rely on site-specific data, rather than adopting values from the literature. Although most design methods neglect it, the presence of a subsurface flow results in an enhancement of the performance of the system. Thermal dispersion also enhances the efficiency of the system but, since it has not yet been adequately studied, relying on it is not advised for the design of BHE fields

Modelling thermal recycling occurring in groundwater heat pumps (GWHPs)

The performance of a Ground Water Heat Pump (GWHP) is often impaired by the thermal recycling between the injection and the extraction well(s), and hence this phenomenon should be evaluated in the design of open loop geothermal plants. The numerical flow and heat transport simulation of a GWHP requires an expensive characterization of the aquifer to obtain reliable input data, which is usually not affordable for small installations. To provide a simple, fast and inexpensive tool for preliminary and sensitivity analyses, an open-source numerical code was developed, which solves the hydraulic and thermal transport problem of a well doublet in the presence of a subsurface flow. The code, called TRS (Thermal Recycling Simulator), is based on a finite-difference approximation of the potential flow theory. The method was validated through the comparison with flow and heat transport simulations with FEFLOW. Subsequently, TRS was run with different values of the aquifer and plant parameters. The correlation observed between some characteristic non-dimensional quantities permitted an empirical correlation to be developed, that describes the time evolution of the extracted water temperature. An example is given for the use of the numerical code and the formula in the dimensioning of an open loop geothermal plant

G.POT: A quantitative method for the assessment and mapping of the shallow geothermal potential

GSHPs (Ground source heat pumps) exchange heat with the ground to provide sustainable heating or cooling. Their technological feasibility and economic viability depend on the site-specific thermal properties of the ground and on the usage profile of the plant. These parameters influence the shallow geothermal potential, which is defined as the thermal power that can be efficiently exchanged by a BHE (Borehole Heat Exchanger) of a certain depth. We present a general method (G.POT) for the determination of shallow geothermal potentials. This method was derived using a comprehensive set of analytical heat transfer simulations, performed by varying (i) the thermal properties of the ground, which comprise its thermal conductivity and capacity, (ii) the thermal properties of the borehole, and (iii) the operational and design parameters of the plant, namely, the BHE length, the threshold temperature of the heat carrier fluid, the duration of the heating/cooling season and the simulated lifetime. Therefore, the G.POT method is a simple and flexible tool that can be implemented in a wide range of different scenarios for large-scale mapping of geothermal potentials. We also assess G.POT by discussing its application to map the geothermal yield in the Province of Cuneo (Piemonte, NW Italy)

Territorial Analysis for the Implementation of Geothermal Heat Pumps in the Province of Cuneo (NW Italy)

The efficiency of Geothermal Heat Pumps (GHPs) strongly depends on the site-specific parameters of the ground, which should therefore be mapped for the rational planning of shallow geothermal installations. In this paper, a case study is presented for the potentiality assessment of low enthalpy geothermal energy in the Province of Cuneo, a district of 6900 km2 in Piedmont, NW Italy. The available information on the geology, stratigraphy, hydrogeology, climate etc. were processed and mapped, and conclusions were drawn on the geothermal suitability and productivity of different areas of the territory surveyed

Environmental sustainability of forward osmosis: The role of draw solute and its management.

Forward osmosis (FO) is a promising technology for the treatment of complex water and wastewater streams. Studies around FO are focusing on identifying potential applications and on overcoming its technological limitations. Another important aspect to be addressed is the environmental sustainability of FO. With the aim to partially fill this gap, this study presents a life cycle analysis (LCA) of a potential full-scale FO system. From a purely environmental standpoint, results suggest that significantly higher impacts would be associated with the deployment of thermolytic, organic, and fertilizer-based draw solutes, compared to more accessible inorganic compounds. The influent draw osmotic pressure in FO influences the design of the real-scale filtration system and in turn its environmental sustainability. In systems combining FO with a pressure-driven membrane process to recover the draw solute (reverse osmosis or nanofiltration), the environmental sustainability is governed by a trade-off between the energy required by the regeneration step and the draw solution management. With the deployment of environmentally sustainable draw solutes (e.g., NaCl, Na2SO4), the impacts of the FO-based coupled system are almost completely associated to the energy required to run the downstream recovery step. On the contrary, the management of the draw solution, i.e., its replacement and the required additions due to potential losses during the filtration cycles, plays a dominant role in the environmental burdens associated with FO-based systems exploiting less sustainable draw solute, such as MgCl2

Groundwater monitoring at a building site of the tidal flood protection system “MOSE” in the Lagoon of Venice, Italy

To protect Venice against tidal flooding, the MOSE system (Experimental Electro-mechanic Module) has been under construction since 2003. This safeguarding system is composed of four batteries of mobile barriers at the Lagoon’s inlets (Lido, Malamocco, Chioggia), which will be lifted before the occurrence of exceptional high tides (>1.10 m above the mean sea level), isolating the Venetian Lagoon from the sea. The end of the construction work is foreseen by 2016. In this paper, the results of the groundwater monitoring at the building site of Punta Sabbioni at the Lido inlet are described. A large dewatered basin (tura), formerly occupied by the sea and close to the shoreline, was used for the precasting of the mobile barriers, and the impact of groundwater control was therefore monitored in the phreatic and shallow confined aquifers. Although a slurry wall barrier was excavated to isolate the tura, a drawdown cone in the confined aquifer was observed, extending to 1 km from the construction site. In contrast, the phreatic aquifer was only influenced by tides, rainfall and evapotranspiration, and the slurry wall of the tura had a positive effect of decreasing the groundwater salinity by limiting the seawater intrusion, as confirmed by the electrical conductivity profiles measured inside the piezometers. The monitoring activity was successful in assessing the impacts of the construction work on the aquifer system and in distinguishing them from the effects of natural driving forces

Environmental and Economic Impact of the Antifreeze Agents in Geothermal Heat Exchangers

Borehole heat exchangers (BHEs) generally employ water-antifreeze solutions to allow working fluid temperatures to fall below 0 °C. However, some local regulations have forbidden antifreeze additives (even non-toxic ones) to avoid groundwater pollution in case of pipe leakage. This paper presents a techno-economic and environmental analysis of four different fluids: propylene glycol at 25% and 33% weight concentrations, calcium chloride at 20% weight concentration (CaCl2 20%), and pure water. Thermal loads from 36 case studies in six different climate zones are used to perform BHE sizing and compare the abovementioned fluids from the economic, operational, and environmental points of view. The economic analysis and the carbon footprint assessment are performed on a life cycle of 25 years considering the installation (BHE drilling, fluid) and operation (heat pump and ground-side circulation pump energy demand, fluid replacement) of the simulated GSHPs. Results highlight that using pure water as a heat carrier fluid is convenient for cooling-dominated buildings but, for heating-dominated buildings, this choice leads to a noticeable increase of the BHE needed length which is not compensated by the lower operational costs. On the other hand, avoiding the use of antifreeze additives generally leads to a reduction of the lifetime carbon footprint, with a few exceptions in very cold climates. CaCl2 20% proves to be a good choice in most cases, both from the economic and the environmental points of view, as it allows a strong reduction of the installed BHE length in cold climates with a low additional cost and carbon footprint

Heat Pumps, Wood Biomass and Fossil Fuel Solutions in the Renovation of Buildings: A Techno-Economic Analysis Applied to Piedmont Region (NW Italy)

The levelized cost of heat (LCOH) and the technical feasibility in the specific context of building construction or renovation are the major drivers of users’ choices for space heating and cooling solutions. In this work, the LCOH was assessed for the most diffused heating technologies in Piedmont (NW Italy): that is, fossil fuels (methane, heating oil and liquefied petroleum gas—LPG), wood biomass (wood logs and pellet) and heat pumps (air-source and ground-source), both in heating-only and in a heating and cooling configuration. A sensitivity analysis of the main LCOH drivers was performed to assess whether and how each technology is vulnerable to energy price and upfront cost changes. The results show that heat pumps are competitive against gas boilers, but they are heavily dependent on refurbishment incentives and penalized by the high electricity prices in Italy; on the other hand, wood biomasses are competitive even in the absence of incentives. The analysis confirmed that LPG and heating oil are no more competitive with renewable heating. Acting on the taxation of natural gas and electricity is key to making heat pumps the most economically convenient solution to cover the heating and cooling needs of buildings

Assessment of energetic, economic and environmental performance of ground-coupled heat pumps

Ground-coupled heat pumps (GCHPs) have a great potential for reducing the cost and climate change impact of building heating, cooling, and domestic hot water (DHW). The high installation cost is a major barrier to their diffusion but, under certain conditions (climate, building use, alternative fuels, etc.), the investment can be profitable in the long term. We present a comprehensive modeling study on GCHPs, performed with the dynamic energy simulation software TRNSYS, reproducing the operating conditions of three building types (residential, office, and hotel), with two insulation levels of the building envelope (poor/good), with the climate conditions of six European cities. Simulation results highlight the driving variables for heating/cooling peak loads and yearly demand, which are the input to assess economic performance and environmental benefits of GCHPs. We found that, in Italy, GCHPs are able to reduce CO2 emissions up to 216 g CO2/year per euro spent. However, payback times are still quite high, i.e., from 8 to 20 years. This performance can be improved by changing taxation on gas and electricity and using hybrid systems, adding a fossil-fuel boiler to cover peak heating loads, thus reducing the overall installation cost compared to full-load sized GCHP systems

Thermal Impact Assessment of Groundwater Heat Pumps (GWHPs): Rigorous vs. Simplified Models

Groundwater Heat Pumps (GWHPs) are increasingly adopted for air conditioning in urban areas, thus reducing CO2 emissions, and this growth needs to be managed to ensure the sustainability of the thermal alteration of aquifers. However, few studies have addressed the propagation of thermal plumes from open-loop geothermal systems from a long-term perspective. We provide a comprehensive sensitivity analysis, performed with numerical finite-element simulations, to assess how the size of the thermally affected zone is driven by hydrodynamic and thermal subsurface properties, the vadose zone and aquifer thickness, and plant setup. In particular, we focus the analysis on the length and width of thermal plumes, and on their time evolution. Numerical simulations are compared with two simplified methods, namely (i) replacing the time-varying thermal load with its yearly average and (ii) analytical formulae for advective heat transport in the aquifer. The former proves acceptable for the assessment of plume length, while the latter can be used to estimate the width of the thermally affected zone. The results highlight the strong influence of groundwater velocity on the plume size and, especially for its long-term evolution, of ground thermal properties and of subsurface geometrical parameter