Numerical Simulation of Nanofluid Suspensions in a Geothermal Heat Exchanger

A Critical Review on the Use of Shallow Geothermal Energy Systems for Heating and Cooling Purposes

The reduction of CO2 emissions has become a global concern. In this regard, the EU intends to cut CO2 emissions by 55% by 2030 compared to those of 1990. The utilization of shallow geothermal energy (SGE) in EU countries is considered the most effective measure for decarbonizing heating and cooling. SGE systems utilize heat energy collected from the earth’s crust to provide secure, clean, and ubiquitous energy. This paper provides a literature review on the use of SGE for heating and cooling purposes. The latest advances in materials, new innovative structures, and techno-economic optimization approaches have been discussed in detail. Shallow geothermal energy’s potential is first introduced, and the innovative borehole structures to improve performance and reduce installation cost is outlined. This is followed by an extensive survey of different types of conventional and thermally enhanced collectors and grouts. Attention is mainly given to the techno-economic analysis and optimization approaches. In published case studies, the least economic break-even point against fossil fuel-based heating systems occurs within 2.5 to 17 years, depending on the local geological conditions, installation efficiency, energy prices, and subsidy. Ground source heat pumps’ cost-effectiveness could be improved through market maturity, increased efficiency, cheap electricity, and good subsidy programs.

Experimental Evaluation of a Full-Scale HVAC System Working with Nanofluid

Nowadays, energy saving is considered a key issue worldwide, as it brings a variety of benefits: reducing greenhouse gas emissions and the demand for energy imports and lowering costs on a household and economy-wide level. Researchers and building designers are looking to optimize building efficiency by means of new energy technologies. Changes can also be made in existing buildings to reduce the energy consumption of air conditioning systems, even during operational conditions without dramatically modifying the system layout and have as low an impact as possible on the cost of the modification. These may include the usage of new heat transfer fluids based on nanofluids. In this work, an extended experimental campaign (from February 2020 to March 2021) has been carried out on the HVAC system of an educational building in the Campus of University of Salento, Lecce, Italy. The scope of the investigation was comparing the COP for the two HVAC systems (one with nanofluid and the other one without) operating concurrently during winter and summer: simultaneous measurements on the two HVAC systems show that the coefficient of performance (COP) with nanofluid increased on average by 9.8% in winter and 8.9% in summer, with average daily peaks of about 15%. Furthermore, the comparison between the performance of the same HVAC system, working in different comparable periods with and without nanofluids, shows a mean increase in COP equal to about 13%.

Nanotechnology Applications in Ground Heat Exchanger Pipes: A Review

The use of Ground Source Heat Pumps (GSHPs) has grown exponentially around the world over recent decades. The GSHP represents an alternative device to electric heating systems and oil boilers. Additionally, it requires a lower power consumption and less maintenance than combustion-based heating systems. Moreover, the CO2 emissions produced by a GSHP are lower than other systems based on burning oil, gas, or biomass. However, the main obstacle for the widespread use of GSHPs is the high cost of Ground Heat Exchanger (GHE) installation, a technology that exhibits low thermodynamic efficiencies. Over the past decade, some studies have been conducted to improve heat transfer in GHE pipes using traditional working fluids, creating new pipe materials or designing new heat exchanger configurations. The main contribution of this paper is a summarization of the outcomes of theoretical, numerical and experimental studies to improve heat transfer in GHEs using nanotechnology. Additionally, the development of new fluids (nanofluids) and new materials (nanoparticles and nanocomposites) applied to heat exchanger pipes and the designs and configurations of GHEs are highlighted. As a result, the present review provides a perspective for future research regarding the use of nanotechnology to reduce the costs involved in GHE for GSHP improvement.

Assessment of Ground Regeneration around Borehole Heat Exchangers between Heating Seasons in Cold Climates: A Case Study in Bialystok (NE, Poland)

Based on the experimental studies, the process of ground regeneration around the borehole loaded with brine-water heat pumps working exclusively for heating purposes in the period of four consecutive heating seasons in a cold climate was presented. The research was conducted in north-eastern Poland. The aim of the work is to verify the phenomenon of thermal ground regeneration in the period between heating seasons on the basis of the recorded data and to check whether the ground is able to regenerate itself and at what rate. It was noticed that the ground does not fully regenerate, especially during heating seasons with lower temperatures. In the analyzed period, from 22 September 2016 to 12 October 2020, the ground probably cooled irreversibly by 1.5 °C. In order to illustrate and evaluate the speed of changes in the ground, the one’s profile with an undisturbed temperature field was presented for each month of the year. The presented results can be a very important source of information for the analysis of geothermal conditions occurring in the ground. They can be used to verify mathematical models and conduct long-term simulations that allow us to see the complexity of the processes taking place in the ground.

Numerical Evaluation of a HVAC System Based on a High-Performance Heat Transfer Fluid

Nanofluids have great potential to improve the heat transfer properties of liquids, as demonstrated by recent studies. This paper presents a novel idea of utilizing nanofluid. It analyzes the performance of a HVAC (Heating Ventilation Air Conditioning) system using a high-performance heat transfer fluid (water-glycol nanofluid with nanoparticles of Al2O3), in the university campus of Lecce, Italy. The work describes the dynamic model of the building and its heating and cooling system, realized through the simulation software TRNSYS 17. The use of heat transfer fluid inseminated by nanoparticles in a real HVAC system is an innovative application that is difficult to find in the scientific literature so far. This work focuses on comparing the efficiency of the system working with a traditional water-glycol mixture with the same system that uses Al2O3-nanofluid. The results obtained by means of the dynamic simulations have confirmed what theoretically assumed, indicating the working conditions of the HVAC system that lead to lower operating costs and higher COP and EER, guaranteeing the optimal conditions of thermo-hygrometric comfort inside the building. Finally, the results showed that the use of a nanofluid based on water-glycol mixture and alumina increases the efficiency about 10% and at the same time reduces the electrical energy consumption of the HVAC system.

Impact of Employing Hybrid Nanofluids as Heat Carrier Fluid on the Thermal Performance of a Borehole Heat Exchanger

In this numerical study, 4 types of hybrid nanofluid, including Ag-MgO/water, TiO2-Cu/water, Al2O3-CuO/water, and Fe3O4-multi-wall carbon nanotube/water, have been considered potential working fluid in a single U-tube borehole heat exchanger. The selected hybrid nanofluid is then analyzed by changing the volume fraction and the Reynolds number. Based on the numerical results, Ag-MgO/water hybrid nanofluid is chosen as the most favorable heat carrier fluid, among others, considering its superior effectiveness, minor pressure drop, and appropriate thermal resistance compared to the pure water. Moreover, it was indicated that all cases of Ag-MgO/water hybrid nanofluid at various volume fractions (from 0.05 to 0.20) and Reynolds numbers (from 3200 to 6200) could achieve better effectiveness and lower thermal resistances, but higher pressure drops compared to the corresponding cases of pure water. Nevertheless, all the evaluated hybrid nanofluids present lower coefficient of performance (COP)-improvement than unity which means that applying them as working fluid is not economically viable because of having higher pressure drop than the heat transfer enhancement.

Determination of the Selected Wells Operational Power with Borehole Heat Exchangers Operating in Real Conditions, Based on Experimental Tests

On the basis of experimental studies, the operational power of four borehole heat exchangers (BHE) under real conditions was determined. The research was carried out in 2018–2019. The theoretical power of the BHE was verified with its operating power. The amount of thermal energy absorbed from the ground by individual BHEs, the operating temperatures obtained at the inlet and outlet of the exchanger, the annual brine flow rate, and the average operating power of the tested wells in two heating seasons were compared and analyzed. Both in 2018 and 2019, none of the examined exchangers achieved an average unit capacity of a well. The aim of the work is to verify the specific ground thermal efficiency indicators adopted for the design of the lower heat source, determined using the computational method and the TRT test with data obtained on the basis of experimental tests. The differences between the results of the tests of the operating parameters of the analyzed BHEs were shown. The data obtained in real conditions is valuable in the research and development of the BHE system.

Application and Design Aspects of Ground Heat Exchangers

With the constant increase in energy demand, using renewable energy has become a priority. Geothermal energy is a widely available, constant source of renewable energy that has shown great potential as an alternative source of energy in achieving global energy sustainability and environment protection. When exploiting geothermal energy, whether is for heating or cooling buildings or generating electricity, a ground heat exchanger (GHE) is the most important component, whose performance can be easily improved by following the latest design aspects. This article focuses on the application of different types of GHEs with attention directed to deep vertical borehole heat exchangers and direct expansion systems, which were not dealt with in detail in recent reviews. The article gives a review of the most recent advances in design aspects of GHE, namely pipe arrangement, materials, and working fluids. The influence of the main design parameters on the performance of horizontal, vertical, and shallow GHEs is discussed together with commonly used performance indicators for the evaluation of GHE. A survey of the available literature shows that thermal performance is mostly a point of interest, while hydraulic and/or economic performance is often not addressed, potentially resulting in non-optimal GHE design.

Performance and Exergy Transfer Analysis of Heat Exchangers with Graphene Nanofluids in Seawater Source Marine Heat Pump System

A marine seawater source heat pump is based on the relatively stable temperature of seawater, and uses it as the system’s cold and heat source to provide the ship with the necessary cold and heat energy. This technology is one of the important solutions to reduce ship energy consumption. Therefore, in this paper, the heat exchanger in the CO2 heat pump system with graphene nano-fluid refrigerant is experimentally studied, and the influence of related factors on its heat transfer enhancement performance is analyzed. First, the paper describes the transformation of the heat pump system experimental bench, the preparation of six different mass concentrations (0~1 wt.%) of graphene nanofluid and its thermophysical properties. Secondly, this paper defines graphene nanofluids as beneficiary fluids, the heat exchanger gains cold fluid heat exergy increase, and the consumption of hot fluid heat is heat exergy decrease. Based on the heat transfer efficiency and exergy efficiency of the heat exchanger, an exergy transfer model was established for a seawater source of tube heat exchanger. Finally, the article carried out a test of enhanced heat transfer of heat exchangers with different concentrations of graphene nanofluid refrigerants under simulated seawater constant temperature conditions and analyzed the test results using energy and an exergy transfer model. The results show that the enhanced heat transfer effect brought by the low concentration (0~0.1 wt.%) of graphene nanofluid is greater than the effect of its viscosity on the performance and has a good exergy transfer effectiveness. When the concentration of graphene nanofluid is too high, the resistance caused by the increase in viscosity will exceed the enhanced heat transfer gain brought by the nanofluid, which results in a significant decrease in the exergy transfer effectiveness.

Is Barocaloric an Eco-Friendly Technology? A TEWI Comparison with Vapor Compression under Different Operation Modes

Barocaloric is a solid-state not-in-kind technology, for cooling and heat pumping, rising as an alternative to the vapor compression systems. The former is based on solid-state refrigerants and the latter on fluid ones. The reference thermodynamical cycle is called active barocaloric regenerative refrigeration (or heat pumping cycle). The main advantage of this technology is to not employ greenhouse gases, which can be toxic or damaging for the environment and that can contribute to increasing global warming. In this paper, the environmental impact of barocaloric technology was evaluated through a Total Equivalent Warming Impact (TEWI) analysis carried out with the help of a numerical 2D model solved through a finite element method. Specifically, we propose a wide investigation on the environmental impact of barocaloric technology in terms of TEWI index, also making a comparison with a vapor compression plant. The analysis focuses on both the cooling and heat pump operation modes, under different working conditions and auxiliary fluids. The results revealed that a barocaloric system based on ABR cycle could provide a reduction of the environmental impact with respect to a vapor compression system. The addition of nanofluids contributes in reducing the environmental impact up to –62%.

Enhancing the Heat Transfer in an Active Barocaloric Cooling System Using Ethylene-Glycol Based Nanofluids as Secondary Medium

Barocaloric cooling is classified as environmentally friendly because of the employment of solid-state materials as refrigerants. The reference and well-established processes are based on the active barocaloric regenerative refrigeration cycle, where the solid-state material acts both as refrigerant and regenerator; an auxiliary fluid (generally water of water/glycol mixtures) is used to transfer the heat fluxes with the final purpose of subtracting heat from the cold heat exchanger coupled with the cold cell. In this paper, we numerically investigate the effect on heat transfer of working with nanofluids as auxiliary fluids in an active barocaloric refrigerator operating with a vulcanizing rubber. The results reveal that, as a general trend, adding 10% of copper nanoparticles in the water/ethylene-glycol mixture carries to +30% as medium heat transfer enhancement.