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Evaluation of a transient borehole heat exchanger model in dynamic simulation of a ground source heat pump system
Abstract
The performance of a vertical ground source heat pump system (GSHPS) largely depends on the fluid temperature leaving the borehole heat exchanger (BHE) that may be affected by the short-term behavior of the BHE. Although considerable research has been carried out to analyze the short-term transient response of the BHEs, few studies have investigated its impact on dynamic simulation of GSHPS. Therefore, this paper presents a numerical approach based on a transient BHE model to evaluate the performance of a residential GSHPS over short and long timescales. The numerical results are compared with the results of EnergyPlus software. It is shown that the proposed model can appropriately predict the dynamic behavior of the system. Moreover, effect of borehole thermal capacity on the performance of the GSHPS is investigated in comparison with a quasi-steady state model. It is found that including the borehole thermal capacity substantially affects the design borehole length. Using the transient model instead of the quasi-steady state model leads to a 16% reduction in the required borehole length.
... Cooling/heating output of GSHP is affected by PLR, defined as follows [45]: ...
... C inco ($•year − 1 ) comes from the infrastructures acquisition and installation funds, so is given as [49]: [45] COP GSHP,ra 4.50 Heating efficiency of GSHP [45] COP GSHP,ra 5.00 Heating efficiency of AH [5] COP AH,ra 1.40 Limited storage range of storge devices [53] β where the subscript i identifies different devices; I represents total number of devices; r denotes the discount rate, which is taken as 6% in this paper [2]; N i indicates the service life of device i, year; c i,inco is the purchase and installation expense of device i, $; P i,in (kW) is the installed capacity of device i, therein, the installed capacity of PV and SC is translated by ideal transformation efficiency and layout area. ...
... C inco ($•year − 1 ) comes from the infrastructures acquisition and installation funds, so is given as [49]: [45] COP GSHP,ra 4.50 Heating efficiency of GSHP [45] COP GSHP,ra 5.00 Heating efficiency of AH [5] COP AH,ra 1.40 Limited storage range of storge devices [53] β where the subscript i identifies different devices; I represents total number of devices; r denotes the discount rate, which is taken as 6% in this paper [2]; N i indicates the service life of device i, year; c i,inco is the purchase and installation expense of device i, $; P i,in (kW) is the installed capacity of device i, therein, the installed capacity of PV and SC is translated by ideal transformation efficiency and layout area. ...
... Energy equipment was modeled with the assumption of constant device efficiency/COP. However, equipment efficiency cannot keep constant in actual operation and would change with operation conditions [12]. Some literature [13,14] DESs. ...
... PLF, defined as the energy efficiency/COP at part load divided by the fullload energy efficiency/COP, is a correction coefficient applied to the steady-state performance of device (0 ≤ PLF ≤ 1) [30]. This method is later adopted in the ASHRAE standard [31], which is also widely employed to evaluate the device performance at part load and has proved valid in previous studies [12,32,33]. The output-input coupling relationship of GB, ASHP and GSHP can be formulated as: ...
... The technical parameters of correlation-based model[10,12,32,33]. ...
With the climate change and depletion of fossil energy, distributed energy systems (DESs) have attracted widespread attention. In this study, a DES driven by solar, geothermal, aerothermal, natural gas and power grid is constructed with energy conversion devices modeled based on part load performance. A novel operation strategy for the DES is presented considering the complementary characteristics of different energy sources. Besides, a multi-objective nonlinear optimization model for the device capacity is proposed with economic, environmental and energy objectives considered simultaneously. To solve the optimization model, an integrated solution method combining Non-dominated Sorting Genetic Algorithm-II, Technique for Order Preference by Similarity to an Ideal Solution and Shannon entropy approach is developed. A case study of an indoor swimming pool in Changsha city of China is undertaken. Optimal equipment capacity and corresponding energy management strategies of the case are obtained. The final number and capacity of air source heat pump (ASHP) are determined via improving its part load ratio. Additionally, three schemes are set to investigate the effects of constant efficiency/COP of energy conversion devices and operation strategies on the capacity optimization of DESs. Results indicate that constant efficiency/COP of equipment yields an 11.7% drop in annual total cost (ATC), a 10.4% increment in annual total CO2 emission (ATE) and a 12.5% reduction in coefficient of energy performance (CEP). ATC and ATE of the optimal solution acquired under a conventional operation strategy increase by 6.8% and 3.7%, while CEP decreases by 66.9%. This work provides a guidance for the future application of DESs.
... Therefore, careful borehole design is essential for the system to meet the reliability needs of long-term operation and to minimise economic costs [22,23]. A typical design problem is to evaluate the total length (or number) of BHEs. ...
... However, in reality, HP operates under partial load most of the time, and the energy efficiency is negatively affected. The part-load fraction (PLF) model can be used to correct the electricity consumption [22]. The equations of the PLF model obtained from the manufacturers are PLF c ¼ 0:18 À 0:21PLR þ 1:03PLR 2 (12) ...
The total length of borehole heat exchangers (BHEs) is an essential issue for ground source heat pump (GSHP) systems. An insufficient number of BHEs may lead to the attenuation of long-term performance and may fail to meet the cooling or heating demands of buildings. Meanwhile, too many BHEs that exceed the system reliability requirement lead to excessive investment. In China, the total length of BHEs calculated using the current Chinese national standard GB50366-2009 is increased when a constant is simply multiplied by engineers; this approach is not economic nor reasonable. Therefore, this study revised the calculation method of the total length of BHEs in Chinese national standards by introducing a parameter called temperature penalty (Tp) to ensure the long-term performance of GSHP systems and minimise the economic cost. A comparison of system reliability under the original BHE length calculation method and the optimised method was performed by simulation. The simulation method was verified through experimental measurement. Regression equations for the thermal conductivity of soil, specific heat capacity of soil, thermal conductivity of grout, daily operation time and Tp were also proposed.
... Therefore, there is now focus on using renewable energy for GSHP hybridization, 60 estimate borehole heat transfer within 8% of experimental values [20]. Finite element modelling 108 has also been carried out using three-dimensional models, which allow for prediction of variable 109 fluid temperatures along the borehole, and can also account for fluid flow dynamics in the borehole 110 pipe [16] [21]. However, in an on-off flowrate GSHP system, calculation of the fluid flow 111 characteristics is not necessary and a three-dimensional numerical model that uses a heat flux term 112 to represent the thermal load of the fluid on the ground has been validated against experimental 113 data [22]. ...
Ground source heat pump systems that are installed in areas with heating or cooling dominant seasons, or in buildings with utilization characteristics that lead to a disparity in demand, often encounter challenges related to ground thermal imbalance. This imbalance can lead to long-term ground temperature changes and may cause premature system failure. This paper focuses on combining a ground source heat pump system with a solar thermal array, with the goal of eliminating the effect of ground thermal imbalance, and minimizing system lifetime cost. A thermal mass ground heat transfer model is combined with a time-stepping model to analyze the system for a variety of solar array sizes. The details associated with this modelling technique are presented, and case studies are provided to illustrate the results of the calculations for three different buildings. It is shown that increasing the solar array size can offset ground thermal imbalances, but increasing the array size also results in a larger initial system cost. An economic analysis is then carried out to determine the system lifetime cost as a function of this solar array size, and an optimal array size from an economic perspective was found. The result of the study shows that hybridizing a ground source heat pump system with a solar array produces a viable system from a technical and economic standpoint, can be used to avoid premature system failure, and can reduce system lifetime cost.
... In this study, the efficiency of the device depended on its nominal or part load and its operating condition, which could be simply determined as a function of PLF and part load ratio (PLR) and expressed by [17,32]: ...
For the carbon-neutral, a multi-carrier renewable energy system (MRES), driven by the wind, solar and geothermal, was considered as an effective solution to mitigate CO2 emissions and reduce energy usage in the building sector. A proper sizing method was essential for achieving the desired 100% renewable energy system of resources. This paper presented a bi-objective optimization formulation for sizing the MRES using a constrained genetic algorithm (GA) coupled with the loss of power supply probability (LPSP) method to achieve the minimal cost of the system and the reliability of the system to the load real time requirement. An optimization App has been developed in MATLAB environment to offer a user-friendly interface and output the optimized design parameters when given the load demand. A case study of a swimming pool building was used to demonstrate the process of the proposed design method. Compared to the conventional distributed energy system, the MRES is feasible with a lower annual total cost (ATC). Additionally, the ATC decreases as the power supply reliability of the renewable system decreases. There is a decrease of 24% of the annual total cost when the power supply probability is equal to 8% compared to the baseline case with 0% power supply probability.
... where PLF is the part load factor; COP ASHP,d is the coefficient of performance under rated working conditions, which is constant from the release data of actual product. The PLF can be calculated according to the different operating characteristics of ASHP [38]: ...
Multi-energy complementary integrated energy system (MCIES) is considered as a promising solution to mitigate carbon emissions and promote carbon peaking and carbon neutrality. Currently, the capacities of a MCIES are sized according to the deterministic load and parameters of the system model. However, uncertainty may lead to the failure to achieve the desired performance and affect the sizing of the MCIES. This study explored an optimization model for the proper sizing of the MCIES considering uncertainties to achieve the best economic, environmental and thermal comfort benefits. The non-dominated sorting genetic algorithm-II (NSGA-II) combined with technique for order preference by similarity to an ideal solution (TOPSIS) and Shannon entropy method were adopted to solve the optimization. Case studies, an actual swimming pool building with MCIES, as the prototype, were used to illustrate the procedure. Moreover, the effects of uncertainty degree and scenario setting were investigated. The results show the benefits of the proposed approach against the traditional deterministic optimization method for comprehensive consideration of economy, environment and thermal comfort. It also suggests that uncertainty and scenario setting should be careful and proper consideration during the design stage, as they have a significant impact on the results of sizing.
... The various parameters considered in this study are soil thermal capacity, grout and ground properties, heat conduction in the fluid, and HTF flowrate. Further, the validation of this model has been done using experimental results [222,223]. FEM has been developed using the parameters, namely, temperature difference of the pipe wall, borehole thermal resistance, with groundwater flow (Al-Khoury et al.) [224] and without groundwater flow (Diersch et al.). Further, the validation of the results has been donewith experimental and numerical data. ...
The research collection aims at finding the various possible opportunities for the effective integration of shallow geothermal energy (SGE) to decrease the energy demand in the built environment and to reduce emission associated with it. The integration of SGE with heat pump using pipe network is extensively reviewed. The open loop and closed loop (vertical, horizontal, energy piles) pipe networks are the most common type of ground heat exchanging methods. The objective of the review is to improve the heat exchanger effectiveness through various design aspects according to the local climatic conditions. This comprehensive review part II contains the research details pertaining to the last two decades about ground heat exchangers (geometrical aspects, borehole material, grout material, thermal response test, analytical and numerical models). Also, the factors influencing the ground heat exchanger's performance such as heat transfer fluid, groundwater flow, and soil properties are discussed in detail. This paper highlights the recent research findings and a potential gap in the ground heat exchanger.
... Well-known technologies capable of guarantying heat exchange between the ground and buildings are ground source heat pump (GSHP) systems [3,4]. The GSHP system consists of a ground heat exchanger which can be buried horizontally [5], vertically [6,7] or slinky [8], a ground heat pump (GHP) to concentrate the temperature level from the ground and an emission system to transfer this energy into buildings. The ground source heat pump (GSHP) system is one of the most efficient system applied in building, this result is widely confirmed worldwide in real installations and by researchers in experimental and numerical studies [9,10]. ...
The endeavor of this paper is to investigate the feasibility of a solar-assisted ground source heat pump (SAGSHP) system for space heating in the climatic condition of Northern Tunisia. Therefore, an experimental set-up was installed and tested in the Research and Technology Center of Energy. The SAGSHP system consists of a ground source heat pump (GSHP) system with a horizontal ground heat exchanger and a buried tank, buried both 1 m deep, and a solar flat-plate collector (SFC) with an area of 2 m² as an auxiliary heat source. Two configurations are set and technically evaluated: (a) space heating by SFC only, and (b) SAGSHP. The results show that: (a) standalone SFC for heating increases the indoor air temperature by about 3 °C, but it cannot guarantee thermal comfort (22 °C), and (b) The tested SAGSHP achieves a significant high performance for space heating.
An exergetic analysis proves high performance of the SAGSHP system, in addition an economic evaluation shows good cost-benefit of the SAGSHP system as compared to other conventional heating systems.
... The GSHPs are fully capable of meeting the cooling demands of buildings in hot climates, specifically, in the Middle East [4]. The most common type of ground heat exchangers is the vertical ground heat exchanger with the length of 100-150 m [5,6]. The empty space between pipe and borehole wall is usually filled with grout which helps to improve the heat transfer rate between circulating fluid inside pipes and ground [7]. ...
Using fins on the inner and outer surfaces of pipes is one method to improve the heat transfer rate of ground heat exchangers (GHEs), thereby reducing the borehole depth and construction and operation costs. Results of 3D numerical studies of simple and finned U-tubes with outer and inner fins are evaluated for GHEs under similar physical conditions. Dynamic and static simulations show the effects of longitudinal fins on the thermal performance of borehole heat exchangers (BHEs) and heat transfer rate between circulating fluid and soil around pipes, while the dynamic tests include short timescale and frequency response tests. The results indicate that the maximum fluid temperature change is about 2.9% in the external finned pipe and 11.3% in the internal finned pipe compared to the finless pipe. The effects of the inlet velocity on temperature profiles, the patterns of the velocity and temperature contours due to the borehole curvature and the response times of the systems under various frequencies are also investigated in detail.
... The results showed that the overall system COP and exergetic efficiency increased with an increase in the heat source temperature and a decrease in the reference environment temperature. To evaluate the performance of a residential GSHP over different timescales, Biglarian et al. [22] presented a numeric model integrated the thermal resistance-capacity model, the equation-fitting model, and the part-load fraction model. The proposed model was validated by EnergyPlus software and can accurately predict the dynamic performance of the system. ...
This study implemented the performance assessment and techno-economic optimization of residential ground source heat pump (GSHP). First, a detailed numerical model integrating the ground heat exchanger and heat pump and economic model considering economic indicators were developed to analyze the thermal and economic efficiency of residential GSHP. Then, Taguchi method was proposed to investigate the impacts of five parameters on the thermal and economic performance of GSHP and obtain the optimal parameters set. After that, the sensitivity of optimal parameter to the drilling costs, pipe prices, and interest rate were performed. Lastly, the analysis of variance approach was implemented to find out the relative significance of parameters. Results revealing that the superiority of individual GSHP can be maintained in the vast majority of time and the capital investments are relatively high within the range of 13034.4 to 18199.2 CNY. The optimal parameters combination was of great discrepancies when taking the economic indicators into account. The most economical choice of pipe size remains unchanged, despite the optimization objectives and varying pipe prices. The relative importance of fluid velocity, borehole depth, and pipe material towards the variation of economic efficiency largely depends on the drilling costs, pipe prices, and the interest rate.
... Results showed non-negligible impact of dynamic operation characteristics and system accessories in the prediction of energy consumption. Biglarian et al. [11] evaluated the performance of a residential GSHPs over short and long time-scales based on a dynamic borehole GHE model. The effect of borehole thermal capacity on the GSHP performance was then compared against a quasi-steady state model, according to which the borehole thermal capacity substantially affected the required borehole length. ...
We present the dynamic thermal analysis of a residential ground-source heat pump (GSHP) with the following objectives: (1) to present a comprehensive GSHP model accounting for the dynamic behavior of a vapor compression unit, lumped serviced space, and the ground heat exchanger (GHE); (2) to identify a set of GHE temperature, mass flow rate, and evaporator area fraction ($\alpha$) improving the time-averaged coefficient of performance (COP) while retaining thermal comfort in the serviced space; and (3) to propose a GHE design satisfying the desired design and operating conditions determined in (2). For a 4-ton residential GSHP considered herein, we verified the existence of a critical GHE mass flow rate of 1 kg/s after which the COP hardly varied. Furthermore, we observed a design trade-off between cooling and heating modes wherein smaller $\alpha$ was preferred in cooling mode while the heating mode favored higher $\alpha$. Cooling COP was sensitivity to both $\alpha$ and GHE temperature ($T_B$), whereas that in heating mode was insensitive to $T_B$ as long as both $\alpha$ and $L_t$ were sufficiently high. The optimal $\alpha$ impartial to both modes is 0.29 and the peak COP=5.49 and 3.6 in cooling and heating mode, respectively.
... Researches on GSHPs are mainly related to GSHP design guidance and heat exchanger simulations [18,19] or to system application effects and control strategies [20,21]. The most common type of ground heat exchanger is the Borehole Heat Exchanger (BHE), a vertical pipe loop reaching depths of 50-200 m [22,23]. ...
In the European Union (EU), buildings are responsible for about 40% of the total final energy consumption, and 36% of the European global CO2 emissions. The European Commission released directives to push for the enhancement of the buildings energy performance and identified, beside the retrofit of the current building stock, Heating, Ventilation, and Air Conditioning (HVAC) systems as the other main way to increase renewable energy sharing and overall building energy efficiency. For this purpose, Ground Source Heat Pumps (GSHPs) represent one of the most interesting technologies to provide energy for heating, cooling, and domestic water production in residential applications, ensuring a significant reduction (e.g., up to 44% compared with air-source heat pumps) of energy consumption and the corresponding emissions. At present, GSHPs mainly employ the refrigerant R410A as the working fluid, which has a Global Warming Potential (GWP) of 2087. However, following the EU Regulation No. 517/2014 on fluorinated greenhouse gases, this high GWP refrigerant will have to be substituted for residential applications in the next years. Thus, to increase the sustainability of GSHPs, it is necessary to identify short time alternative fluids with lower GWP, before finding medium-long term solutions characterized by very low GWP. This is one of the tasks of the UE project "Most Easy, Efficient, and Low-Cost Geothermal Systems for Retrofitting Civil and Historical Buildings" (acronym GEO4CIVHIC). Here, a thorough thermodynamic analysis, based on both energy and exergy analysis, will be presented to perform a comparison between different fluids as substitutes for R410A, considered as the benchmark for GSHP applications. These fluids have been selected considering their lower flammability with respect to hydrocarbons (mainly R290), that is one of the main concerns for the companies. A parametric analysis has been performed, for a reversible GSHP cycle, at various heat source and sink conditions, with the aim to identify the fluid giving the best energetic performance and to evaluate the distribution of the irreversibilities along the cycle. Considering all these factors, R454B turned out to be the most suitable fluid to use in a ground source heat pump, working at given conditions. Special attention has been paid to the compression phase and the heat transfer in evaporator and condenser.
... The results show that both the seepage velocity and flow velocity in the wellbore influence the heat transfer of a GSHP system. A transient GSHP model (Biglarian et al., 2018) has been used to investigate the impact of short and long timescales on dynamic performance, and the simulation results are a reference for optimizing the design length of a borehole. Groundwater flow can be considered to optimize the cost of the GCHP system, by using G-functions based on analytical models (Samson et al., 2018). ...
Shallow geothermal energy is stable and clean. Using a heat pump to produce groundwater and realize heating and cooling can effectively prevent haze and reduce energy consumption. To reduce engineering costs, many buildings in Beijing, China, plan to utilize single-well groundwater source heat pumps. Numerical modeling is an effective way to gain an understanding of thermal transport processes. However, wellbore-reservoir coupling and the uncertainty of productivity due to geological parameters make simulation difficult. A wellbore-reservoir-integrated fluid and heat transport model is defined by T2Well simulator to predict the productivity of a typical single-well system, with consideration of complex geological factors. The model is validated by the analytical model developed in Beijing, China. The fluid processes in the wellbore are described by 1 D non-Darcy flow, and the reservoir 3 D fluid and heat transport processes are calculated. Six crucial factors satisfying a random distribution are used, and for a single well that can supply heat for an area of 9000 m², the output temperature during the heating season ranges from 11°C to 15°C.
... When the borehole heat exchangers coupled to the GSHP have to be sized, the overall energy demand of the building and the energy profiles for both heating and cooling conditions are the primary input data. There are different methodologies to design a GSHP: (a) The analytical models (e.g., the infinite line or cylindrical source model) [10-12], (b) the method based on the g-functions [13][14][15], and (c) numerical models (e.g., [16][17][18]). Depending on the methodology used, the heating/cooling energy demand of the building has to be provided by the following data: ...
The design of ground source heat pumps is a fundamental step to ensure the high energy efficiency of heat pump systems throughout their operating years. To enhance the diffusion of ground source heat pump systems, two different tools are developed in the H2020 research project named, “Cheap GSHPs”: A design tool and a decision support system. In both cases, the energy demand of the buildings may not be calculated by the user. The main input data, to evaluate the size of the borehole heat exchangers, is the building energy demand. This paper presents a methodology to correlate energy demand, building typologies, and climatic conditions for different types of residential buildings. Rather than envelope properties, three insulation levels have been considered in different climatic conditions to set up a database of energy profiles. Analyzing European climatic test reference years, 23 locations have been considered. For each location, the overall energy and the mean hourly monthly energy profiles for heating and cooling have been calculated. Pre-calculated profiles are needed to size generation systems and, in particular, ground source heat pumps. For this reason, correlations based on the degree days for heating and cooling demand have been found in order to generalize the results for different buildings. These correlations depend on the Köppen–Geiger climate scale.
... It is possible to install these systems everywhere; therefore, in outland areas such as small villages that energy transfer in relatively high scale is too expensive, these systems lead to more affordable cost of heating or cooling. In addition, thermal energy efficiency and exergy efficiency of these systems are higher in comparison with conventional systems and due to negligible change of ground temperature during the year, the operations of GSHP systems are almost stable [12][13][14][15]. ...
Energy and exergy analyses and thermo-economic optimization of a geothermal (ground source) heat pump for domestic water heating are performed in this study. The modeling and optimization of the geothermal heat pump with closed horizontal ground heat exchanger are conducted by applying EES program. The effect of water temperature at the inlet of evaporator is investigated on the coefficient of performance (COP) for several types of refrigerants. The results show that R507a has the highest COP in comparison with the other refrigerants. In the optimization process, the total annual cost (TAC) is selected as objective function, and optimum design parameters are calculated for minimizing the defined objective function. Moreover, variations of TAC values with heating capacity, and overall heat transfer coefficient of soil for various refrigerants for three climatic regions of Iran are discussed. The results show that by changing the temperature of water entering the evaporator, R507a has the lowest TAC value. Also, the greatest share of the system cost is corresponded to ground heat exchanger which is 34 % of total cost. In addition, the compressor has the maximum irreversibility of the system about 53%. Also, variations of TAC value at low temperatures of inlet water are almost equal and with smooth slope for three cities of Tehran, Yazd and Rasht. However, at high temperatures, the slope variation in Yazd is lower in comparison with Tehran and Rasht. Additionally, at optimum conditions, the saturation temperatures for evaporator and condenser are estimated −1.5 and 51.89°C, respectively.
... On the other hand, the thermal performance of the solar collector is calculated using the Hottel-Whillier-Bliss equation. The detailed numerical approach for dynamic simulation of the integrated GSHP system was recently presented by Biglarian et al. (2018). ...
In recent years, increasing attention has been paid to the use of renewable energy sources in building sector. The ground source heat pumps are a promising alternative for heating and cooling applications. Despite the high coefficients of performance and significant environmental benefits, their performance may deteriorate over the long-term operation if the annual ground loads are not well balanced. An appropriate solution is the use of hybrid ground source heat pump system through which some portion of the thermal loads can be offset by supplemental components. The aim of this study is to assess the feasibility of a solar-assisted ground source heat pump system for heating a detached house in a cold climate of northwestern Iran. The economic and environmental aspects of the proposed hybrid system are compared with those of a ground source heat pump, an air source heat pump, and a natural gas space heater for a 20-year design life. The analyses show that despite having high initial cost, the hybrid system has the second lowest life cycle cost and the lowest carbon dioxide emissions.
The deep borehole heat exchanger (DBHE) coupled ground source heat pump (GSHP) system is widely developed in recent years to extract geothermal energy for building heating. However, its operating mechanism and energy flows within different components have not been comprehensively investigated. Meanwhile, the high outlet temperature of DBHE has the potential to meet the direct heating requirement but may exceed the GSHP’s temperature threshold. Therefore, this study first presented a hybrid DBHE heating system transient model with direct heating and coupled GSHP approaches using TRNSYS and MATLAB software. The novelty of this system is that DBHE was also coupled with a heat exchanger (HEX) to realize direct heating. Taking a residential building in Xi’an and DBHE with the depth of 2000 m as a scenario, the operation behavior and energy performance of the system were analyzed. The results showed that HEX will operate for several days before operating GSHP at the beginning of heating season, and annual HEX operating time gradually decreases from 27.1 days to 8.8 days over 10 years. Annual energy fraction of heat extracted from the soil by the HEX and GSHP in the total heating demand gradually decreases from 0.14 to 0.05 and increases from 0.72 to 0.78 over 10 years, respectively. Further, the importance of HEX in system performance was evaluated. Compared with the hybrid system, the GSHP’s COP will be overestimated by 11.3 % and the heat extraction amount will be significantly underestimated when HEX is not designed in the DBHE system. Moreover, the effects of different building heating load characteristics (including actual load and constant load) on system performance were discussed. It was found that the minimum inlet temperature of DBHE during the heating season under constant load will be overestimated, with a maximum deviation of 8.4 °C. Dynamic building load also results in a parabolic soil temperature isotherm at the end of heating season.
The growing demand for energy and the pressure of carbon emission increase the proportion of renewable energy in the global energy consumption. In the wind-to-heat system, the compressor is utilized to replace the generator in the wind turbine to allow the wind energy driving heat pump directly, which simplifies the energy conversion process. In this study, a dynamic simulation model of the wind-to-heat system is proposed, which consists of the following six parts: wind turbine, compressor, condenser, thermal expansion valve, evaporator, and controller. Validations of this model was conducted by comparing the simulation result with the experimental data from a 100 kW wind-to-heat system. Effects of different operational conditions, such as hot water flow rate, inlet water temperature and types of refrigerants, on the characteristics of the system are explored and analyzed. The conclusion is drawn that the hot water flow rate is optimized as 3 kg/s, lower inlet water temperature led to lower heating capacity and lower COP, R245fa and R717 have good performances in heating production, which is in favor of the further design and optimize of the system. After debugging the simulation model for different design alternatives, different performances will be acquired to improve the efficiency of the design.
The application of ground source heat pump (GSHP) system for green buildings is an effective way to improve the energy efficiency of buildings and reduce energy demand. At present, the GSHP system for green buildings has the problem of improper operation management and unideal operation effect. To comprehensively analyze the actual operation of system and promote the healthy development of GSHP system for green buildings, it is necessary to establish a comprehensive, quantitative and targeted application effect evaluation system. In this paper, the evaluation indexes were selected from three aspects of standards and specifications, literature researches and field investigations. Based on the evaluation purposes of application effect evaluation, operation optimization guidance and investment decision reference, the targeted evaluation indexes were constructed. The hierarchical-entropy weight method was used to determine the index weight according to the results of expert investigations and typical case measurements. Then, a fuzzy comprehensive evaluation model was established to integrate the multi-index evaluation results to form a complete evaluation system. The system covers 21 quantitative evaluation indexes, and results show that the evaluation indexes with the largest weight in the criterion, sub-criterion and index layers were technicality (0.69), system performance (0.33) and system heating performance coefficient (0.12), respectively. The evaluation system of the application effect of GSHP system for green buildings is useful for evaluating the application effect of system and convenient for operators to identify the weak points of system operation and adjust the operation schemes in time. Such an evaluation system can be used as a valuable reference for investors' investment decisions.
With the continuous expansion of computer system simulation applications, the dynamic system modeling problems that people face have become more and more complex. This paper mainly studies dynamic system simulation models and algorithms based on artificial neural networks. This article mainly focuses on some practical problems of dynamic system simulation, and studies the artificial neural network model and corresponding algorithm suitable for dynamic system modeling. First, it summarizes the existing artificial neural network simulation models of dynamic systems: multi-layer feedforward network relationship model without feedback, recursive network relationship model with feedback, and their network topology diagrams are drawn. Secondly, a quantum genetic algorithm is proposed to optimize the parameters of artificial neural networks and feedforward process neural networks. To achieve the preprocessing of the parameters. Next, based on the analysis and research of BP neural network theory, this paper comprehensively compares the neural network models established under various network structures and algorithms, and establishes the simulation model based on the BP neural network monitoring data system, and compares the results with the monitoring data obtained in the field. The hidden layer of neural network with 4 and 6 nodes will increase the convergence step. When the number of nodes is 10 and 12, the convergence rate may reach 46 steps. This shows that if the non-display layer with different number of nodes in the same layer is selected for the same system simulation model based on artificial neural network, the repetition number of convergence steps of the prediction model will be different.
A solar-assisted natural gas distributed energy system (DES) with energy storage is proposed to determine the optimal configuration of the DES in this study. A mixed-integer nonlinear programming (MINLP) model is established considering the part-load performances of devices and the annual total cost (ATC) as objective. Taking an energy center in Jinan, China as the objective building, three scenarios of different structural DESs are designed in which different candidate devices of a certain type of equipment are listed, and the optimal configurations and the energy management strategies are studied. The results show that in comparison with the conventional DES (Scenario 1), the ATCs of the DES with solar energy (Scenario 2) and the solar-assisted natural gas DES with energy storage (Scenario 3) are decreased by 2.90% and 7.48%. The economy and flexibility of the power grid is verified to regulating the electrical load of DES. The sensitivities of the DES to energy price changes are analyzed and the DES is more sensitive to the natural gas price under the research ranges. The model and method can be applied to practical engineering and give guidance on device operation.
Ground source heat pump (GSHP) system is an efficient solution to mitigate energy consumption and environmental emissions in the building sector. Although the GSHP system is categorized among renewable energy technologies, considering the fact that the system's power demand is usually provided by fossil fuel resources, it may not fully meet this classification. One way to offset the GSHP's electricity consumption in a renewable approach is the use of photovoltaic (PV) panels. This paper aims to study the feasibility of a hybrid PV-GSHP system for a residential building in Tehran, Iran. To that end, a numerical model is developed to explore the GSHP system's performance over a 20-year design life, taking several borehole lengths into account. Moreover, the PV modules are simulated by EnergyPlus software. A life cycle cost analysis is also performed to assess the economic viability and optimal design of the PV-GSHP system. The results illustrate that using four PV modules can best cover the system's electricity consumption and the discounted payback period is less than four years. Furthermore, an environmental evaluation reveals that the PV-GSHP system can save 29.2 t of carbon dioxide emissions over the 20-year period compared to the GSHP system.
For ground source heat pumps with boreholes, the rule-of-thumb and thermal performance test (TPT) methods are commonly adopted for borehole design but suffer from inaccuracy. The response factor model can calculate the time-varying soil heat transfer of boreholes fast and accurately. Based on the response factor model, the characteristic diagrams of borehole heat transfer are summarized to show the influential laws of different factors (borehole inlet temperature, velocity of fluid inside the pipe, initial soil temperature, soil thermophysical properties, and operation time). The characteristic diagrams can be used as a new design method that improves the accuracy of the rule-of-thumb method by considering different influential factors and avoiding the subjective errors of designers. For different soils, the borehole design errors of the new diagram method are -5 % ∼ 10 %, which are much less than the errors (-15 % ∼ 72 %) of the conventional rule-of-thumb method. In addition, a modification to the TPT method is also proposed using the response factor model. The borehole design errors of the conventional TPT method (48h testing) can be reduced by 13 % ∼ 20 % using the derived correction factors. The proposed modification methods contribute to more reasonable borehole design, improved heating reliability as well as increased energy efficiency.
We propose a novel multilateral-well coaxial closed-loop geothermal system (CCGS) to help realize the commercial exploitation of hot dry rock. Compared with the enhanced geothermal system, it avoids the high-cost fracturing and environmental problems by closed-loop circulation. Also, the multilateral-well CCGS can significantly increase heat production than traditional vertical-well or horizontal-well CCGSs via lateral wellbores. Then, we establish a new 3D transient model and validate it with experimental data. The temperature field is studied. The Influences of injection flow rate, number of lateral wellbores, wellbore size, thermal conductivity of insulation pipe, and reservoir temperature are analyzed. Results show that the lateral wellbores are vital to improving heat extraction, and a viable lateral-wellbore spacing is determined. The total energy output rises linearly with time, while the extracted heat in the initial stage is limited though the thermal power is high. There is an appropriate injection flow rate by comparing the total energy output and dissipation. A smaller wellbore size is preferred due to the minor effect. A reasonable thermal conductivity exists for the insulation pipe. Particularly, the high-temperature geothermal resources at various depths can be efficiently developed through the multilateral-well CCGS given the effect of reservoir temperature.
Based on a quasi 3-D steady model inside and a 2-D unsteady model outside of the borehole of ground heat exchanger(GHE), the relationship between the heat exchange rate per depth of borehole and main parameters (center-to-center distance of two branch pipes, radius and depth of the borehole) has been analyzed. The rational values of those design parameters with considering the heat exchange rate per depth of borehole are obtained. The dynamic simulations of underground temperature field (UTF) of single-pipe and pipe-group of U-tube GHE have been conducted by ANSYS. The change rules of UTF are obtained when ground source heat pump(GSHP) has experienced a complete cooling and heating cycle during one year. The simulation results of a single-pipe show that the action radius of borehole is 1.9 m, i.e., it is reasonable that the borehole center-to-center distance is 3.8 m. The simulation results of pipe-group indicate that temperature of the soil center encircled by four boreholes at the beginning of heating in winter is higher than the initial temperature of soil, but it is in opposite for cooling in summer, which is beneficial to the efficient operation of GSHP. This work can provide guidance for the rational design and operation of GSHP.
This paper presents the background, development and the validation of new analytical and numerical solutions for the modeling of short-term response of borehole heat exchangers. The new analytical solution studies the borehole's heat transfer and the related boundary conditions in the Laplace domain. A set of equations for the Laplace transforms for the boundary temperatures and heat-fluxes is obtained. These equations are represented by a thermal network. The use of the thermal network enables swift and precise evaluation of any thermal or physical setting of the borehole. Finally, very concise formulas of the inversion integrals are developed to obtain the time-dependent solutions. The new analytical solution considers the thermal capacities, the thermal resistances and the thermal properties of all the borehole elements and provides a complete solution to the radial heat transfer problem in vertical boreholes. The numerical solution uses a special coordinate transformation. The new solutions can either be used as autonomous models or easily be incorporated in any building energy simulation software.
Common approaches to the simulation of Borehole Heat Exchangers (BHEs) assume heat transfer in circulating fluid and grout to be in a quasi-steady state and ignore fluctuations in fluid temperature due to transport of the fluid around the U-tube loop. Such effects have been shown to have an impact on peak temperatures and hence operation of heat pumps systems when short time scales are considered. A model has been developed that combines a two-dimensional numerical model and models of the pipe loop components. A novel heat exchanger analogy is employed to calculate the heat exchanger outlet temperatures such that iterative procedures can be avoided and numerical stability is unconditional. These approaches result in a model that is computationally efficient and captures much of the short timescale dynamic effects represented in fully three-dimensional models. This is demonstrated by comparison with experimental data and by comparing two and three-dimensional model behaviour in the frequency domain. Predicted monthly outlet temperatures and heat transfer rates are furthermore shown to be in close agreement with experimental values and in good agreement with existing borehole heat exchanger models. The model is computationally efficient enough to allow use in routine analysis and design tasks.
Abstract
A one-dimensional transient ground heat exchanger model is proposed to account for fluid and grout thermal capacities in borehole ground heat exchangers with the objective of predicting the outlet fluid temperature for varying inlet temperature and flow rate. The standard two-pipe configuration is replaced with an equivalent geometry consisting of a single pipe and a cylinder core filled with grout. Transient radial heat transfer in the grout is solved numerically while the ground outside the borehole is treated analytically using the cylindrical heat source method. The proposed model is validated successfully against analytical solutions, experimental data, a three-dimensional transient numerical model, and TRNSYS's Type 451.
For a typical two-pipe configuration, it is shown that the fluid outlet temperature predicted with and without borehole thermal capacity differs by 1.4, 0.35, and 0.23 °C after 0.1, 0.2 and 1 h, respectively. Annual simulations are also performed over an entire heating season (5600 h) with a 6 min time step. Results show that the outlet fluid temperature is always higher when borehole thermal capacity is included. Furthermore, the difference in fluid outlet temperature prediction with and without borehole thermal capacity increases when the heat pump operates infrequently. The end result is that the annual COP predicted is approximately 4.5% higher when borehole thermal capacity is included.
Accurate modeling of heat transfer in the ground and inside the borehole is crucial to correctly size and assess performance of ground coupled heat pump systems. The model proposed here uses a hybrid approach combining two techniques. First, the rapid transient behavior inside the borehole is handled numerically with a fine grid in combination with a model size reduction technique to reduce computation time. Secondly, the surrounding ground is modeled using modified g-functions. The resulting HR (hybrid reduced) model is implemented as a TRNSYS type using a load aggregation algorithm. Results show that differences between the proposed model and a well-known non-capacity model are within an acceptable range of the order of ±0.8 °C. The differences are partly attributed to the simplification methods. However, they are mainly due to the fact that the HR accounts for the thermal capacity in the borehole. In simulations over a heating season, the inclusion of borehole thermal capacity results in outlet fluid temperatures that can be up to 2 °C higher than when thermal capacity is not accounted for. In terms of computation time, the HR model is about 37 times faster than a complete hybrid model, but with almost no loss in accuracy.
Common approaches to the simulation of borehole heat exchangers assume heat transfer within the circulating fluid and grout to be in a quasi-steady state and ignore axial conduction heat transfer. This paper presents a numerical model that is three-dimensional, includes explicit representations of the circulating fluid and other borehole components, and so allows calculation of dynamic behaviours over short and long timescales. The model is formulated using a finite volume approach using multi-block meshes to represent the ground, pipes, fluid and grout in a geometrically correct manner. Validation and verification exercises are presented that use both short timescale data to identify transport delay effects, and long timescale data to examine the modelling of seasonal heat transfer and show the model is capable of predicting outlet temperatures and heat transfer rates accurately. At long timescales borehole heat transfer seems well characterized by the mean fluid and borehole wall temperature if the fluid circulating velocity is reasonably high but at lower flow rates this is not the case. Study of the short timescale dynamics has shown that nonlinearities in the temperature and heat flux profiles are noticeable over the whole velocity range of practical interest. The importance of representing the thermal mass of the grout and the dynamic variations in temperature gradient as well as the fluid transport within the borehole has been highlighted. Implications for simplified modelling approaches are also discussed.
Despite the low energy consumption and lower maintenance benefits of ground-source heat pump (GSHP) systems, little work has been undertaken in detailed analysis and simulation of such systems. Long-term transient ground heat transfer significantly affects the performance of these systems. Annual and multi-year simulation consequently becomes an invaluable tool in the design of such systems—both in terms of calculating annual building loads and long-term ground thermal response.The EnergyPlus program, which makes use of variable time-step sizes in its simulation of building systems, was extended to allow multi-year simulations. Models of a water-source heat pump and a vertical borehole ground-loop heat exchanger have been implemented in Energy-Plus. The ground heat exchanger model uses Eskilson's “g-functions” to model response to time-varying heat fluxes and has been extended to include a computationally efficient variable time-step load aggregation scheme. The performance of this model has been compared with an analytical line source approximation. For a steady periodic input that included pulsated heat extraction, the model agreed with the analytical solution to within 2°C. The heat pump model was able to predict power and heat transfer rates over a wide range of operating conditions to within ±10% of published data.Experimental data from the Oklahoma State Hybrid Ground Source Heat Pump Laboratory have been used to validate both the heat pump and ground heat exchanger models. System simulation results were compared with five days of experimental data. The results showed an average error in the predicted ground heat transfer rate of less than 6% and average errors in the predicted heat pump power and the predicted source-side heat transfer rate of less than 3% and 4%, respectively. Using these models, it is possible to represent GSHP systems in a flexible way and examine their performance over the extended periods required for proper analysis.
Common approaches to the simulation of borehole heat exchangers (BHEs) assume heat transfer in circulating fluid and grout to be in a quasi-steady state and ignore fluctuations in fluid temperature due to transport of the fluid around the loop. However, in domestic ground source heat pump (GSHP) systems, the heat pump and circulating pumps switch on and off during a given hour; therefore, the effect of the thermal mass of the circulating fluid and the dynamics of fluid transport through the loop has important implications for system design. This may also be important in commercial systems that are used intermittently. This article presents transient simulation of a domestic GSHP system with a single BHE using a dynamic three-dimensional (3D) numerical BHE model. The results show that delayed response associated with the transit of fluid along the pipe loop is of some significance in moderating swings in temperature during heat pump operation. In addition, when 3D effects are considered, a lower heat transfer rate is predicted during steady operations. These effects could be important when considering heat exchanger design and system control. The results will be used to develop refined two-dimensional models.
Many of the popular building energy simulation programs around the world are reaching maturity — some use simulation methods (and even code) that originated in the 1960s. For more than two decades, the US government supported development of two hourly building energy simulation programs, BLAST and DOE-2. Designed in the days of mainframe computers, expanding their capabilities further has become difficult, time-consuming, and expensive. At the same time, the 30 years have seen significant advances in analysis and computational methods and power — providing an opportunity for significant improvement in these tools.In 1996, a US federal agency began developing a new building energy simulation tool, EnergyPlus, building on development experience with two existing programs: DOE-2 and BLAST. EnergyPlus includes a number of innovative simulation features — such as variable time steps, user-configurable modular systems that are integrated with a heat and mass balance-based zone simulation — and input and output data structures tailored to facilitate third party module and interface development. Other planned simulation capabilities include multizone airflow, and electric power and solar thermal and photovoltaic simulation. Beta testing of EnergyPlus began in late 1999 and the first release is scheduled for early 2001.
The present study develops a new accurate and simple analytical method for modeling of heat transfer in borehole heat exchanger for short time periods. The thermal resistance and capacity circuit is used for derivation of the governing equations inside the borehole for single U-tube and double U-tube configurations. Also, radial heat transfer is taken into account in the ground around the borehole. The Laplace transform is used for solving the governing equations. The effects of the fluid and grout thermal capacities are considered in the present model. Unlike the previous analytical models reported in the literature, the real geometry of the borehole with single and double U-tube configurations is considered in the present model. The results of the present analytical model are compared with reference experimental measurements and numerical simulation results. It is found that the mean fluid temperature and the borehole wall temperatures obtained from the present analytical method are in very good agreement with the reference numerical results.
Geothermal energy is one of the most promising renewable resources and an alternative to conventional fossil fuels. Downhole heat exchanger is the most common form used for exploitation. Based on the geological data of Xinji geothermal field, China, the entire flow field of a single U-tube downhole heat exchanger inside the borehole is analyzed comprehensively by a three-dimensional steady state model. Thermo-physical properties of working fluid are considered specifically. The performance of the exchanger is studied through investigation on the influences of mass flow rate, degree of depth, length of the U-tube and temperature difference between working fluid and geothermal field. Finally, conversion rates are calculated and comparison of influences between four parameters has been made quantitatively via grey correlation analysis. Simulation values are validated by results obtained from field tests. For geothermal fields with high temperature, it is better to apply a large volume of circulating water to obtain higher heat extraction rate. The length of the U-tube is the most significant impact factor on the conversion rate, with the following sequence being temperature difference, mass flow rate and degree of depth. Therefore, lengthening the U-tube is the most efficient and cost-effective way for the exploitation of geothermal energy.
This paper uses composite cylindrical heat-source (CCS) models and typical thermal response test procedures to investigate two full-scale borehole heat exchangers (BHE); where one is a U-tube BHE, and the other, a pipe-in-pipe (coaxial) BHE. A previously developed CCS model is compared to a simplified infinite line-source (ILS) model. A time-varying heat-flux term is verified for the U-tube case, noting the error found when using the CCS model. A model is developed using a similar approach accounting for a coaxial configuration showing a root mean square error (RMSE) of less than 0.1 °C over the duration of the test.
This paper shows the evaluation of the performance of a ground-coupled heat pump system monitored building providing heating, ventilating and air conditioning to an office building located in Madrid, in Spain. The system consists of one borehole exchanger, heat pump unit, radiant floor system, mechanical ventilation and data control system. A simulation model was performed with EnergyPlus software and validated. The analyzed period corresponds to the most unfavorable weather conditions in heating and cooling mode. The coefficient of performance obtained in heating and cooling mode was 3.86/5.29, considering all the energy consumption elements of the building and the thermal demand corresponding to an office operation. The CO2 emissions obtained with a value of 34.68 kg corresponding to the period analyzed represents a low CO2 emission system. The monitored temperatures reached set point values of 22 °C/25 °C, considered as acceptable comfort temperatures. The values obtained in the validated simulation model presented a deviation of 2% respected experimental results in heating and cooling mode. A comparative of COPsys and CO2 emissions with other technologies is performed in order to analyze GCHP compared to other available technologies. The GCHP system is presented as a technology that can fully supply the HVAC conditions for a building and environmentally friendly.
A numerical model is developed to simulate the borehole heat exchanger both in the short and long time. In this regard, the computational domain is divided into the inside and outside borehole regions. A two-dimensional finite volume method is implemented in a cylindrical coordinate system for modeling of the outside borehole. Also, a thermal resistance-capacity model is presented for the borehole cross section. This model is extended to take into account the fluid transport through the U-tube and the temperature variation of the borehole components with depth. The governing equations of the two regions are solved iteratively in each time step. The proposed model is verified with the previously reported numerical, experimental and analytical results. Furthermore, the ability of the model in predicting the short-time response is evaluated in comparison with a three-dimensional computational fluid dynamics (CFD) model with a fine grid. The results show that the proposed model has a good performance in the prediction of the thermal response of the borehole in a wide time interval from one minute to over 10 years. Moreover, the effects of time step size and number of capacity nodes on the results are investigated.
Cold thermal energy storage (TES) has been an active research area over the past few decades for it can be a good option for mitigating the effects of intermittent renewable resources on the networks, and providing flexibility and ancillary services for managing future electricity supply/demand challenges. In this chapter, three available technologies for cold storage: sensible, latent and sorption storage have been reviewed and summarized from both the materials and application aspects. Issues and possible solutions are introduced and discussed in detail for the storage materials. Cold storage applications can be widened from building and vehicle air conditioning application to fresh and frozen food storage and transport. Sensible storage is a comparatively mature technology that has been implemented and evaluated in many large-scale demonstration plants. Latent storage and sorption have much higher energy storage densities than sensible storage, which are currently still in the stages of material investigations and lab-scale experiments. Heat transfer and performance enhancement are also discussed. These are beneficial for researchers and engineers to design the more cold sustainable thermal systems.
A model has been developed to predict the latent capacity of an air conditioner or heat pump at part-load conditions with a continuously operating supply fan. The model predicts the latent heat ratio (LHR) at part-load conditions as a function of the run-time fraction (X) or cooling-load fraction (CLF). The model parameters necessary to quantify system performance include the maximum cycling rate (Nmax), the air-conditioner time constant (τ), the ratio of the initial evaporation rate and steady-state latent capacity (γ), the nominal time after start-up for condensate removal to begin (twet), and the temperature and humidity entering the coil. The model compared favorably to measured data from Khattar et al. (1985). A sensitivity analysis was conducted to determine the model parameters that have the strongest impact on the part-load LHR. The model can be easily integrated into hourly building simulation models to more accurately predict the dehumidification performance of air conditioners at part-load conditions.
There is an increasing awareness that there are limits to the availability of non-renewable resources, while there is an increasing energy demand throughout the world. This demand is expected to be satisfied through the efficient renewable energy in the near future. However, the world is facing the challenge of variable renewable energy outputs due to a stochastic feature of the energy sources. Thermal energy storage (TES) can be a good option for mitigating the effects of intermittent renewable resources on the networks. It can not only allow the increased renewable energy and night time low price electricity utilization, but also provide flexibility and ancillary services for managing future electricity supply/demand challenges. In this paper, various TES forms, including sensible, latent and sorption are explained and summarized for their performance enhancement. More importantly, from the perspective of sustainability, various integration forms for different applications are systematically introduced, such as TES integration with hot water supply, air conditioners and heat pumps, TES integration with building construction systems, and TES integration with power production cycles, cogeneration, food transport, solar cookers and vehicle systems for thermal comfort. Therefore, this study is beneficial to designing more sustainable thermal systems by the researchers and engineers.
Sensible heat thermal energy storage has been drawing increasing attention for various applications for many years, which is an important technology for solving the time-discrepancy problem of waste or renewable energy utilization. This paper reviews available technologies for sensible heat storage under various operating conditions and storage tank geometries. Several aspects from sensible storage material, water stratification phenomenon, heat storage heat transfer modes, and various influencing factors, have been detailed for introduction. The influencing factors for energy and exergy performance were investigated and summarized from the fluid mass flow rate, storage tank geometrical structure, fluid properties, fluid inlet temperature, etc. The performance comparison with latent thermal storage system was also briefly studied. The paper is beneficial for the researchers and engineers to design more efficient and optimized sensible storage systems.
The short-term behavior of borehole heat exchangers (BHE) is investigated using three 2h test sequences each with constant inlet temperature and constant mass flow rate. Three models of the transient systems simulation program (TRNSYS) have been compared to these measurements. In order to assess the deviations a 3D-FEM model, built in COMSOL Multiphysics®, is compared to the measurement as well.The "Erdwärmesonden" model (EWS) shows a very good accuracy with a deviation less than 10%. The superposition borehole model (SBM) and duct ground heat storage model (DST) neglect the internal borehole heat capacity of fluid and grout. In order to improve their short-term behavior, the models are modified with an adiabatic pipe model with wall capacity in front of the BHE model. Different parameter sets for the pre-pipe are investigated. The optimum parameter set reduces the error between simulation and measurement of the injected heat from 50% to about 5%. With this model modification, all investigated models SBM, DST, and EWS perform as accurate as the COMSOL model.
Latent heat thermal energy storage (TES) can be an efficient option to cater to fluctuating energy demands and at the same time to obtain a higher performance from the energy and exergy aspects. Latent heat TES storage performance is usually influenced by various operating conditions and design parameters during the energy/exergy stored and retrieved. The scope of investigation is to comprehensively review various useful techniques adopted in detail for energy and exergy performance enhancements, and provide the perspectives for researchers and engineers to design more efficient latent TES systems. Various influencing factors can be enlarged to include the heat transfer fluid (HTF) mass flow rate and inlet temperature, phase change material (PCM) melting temperature and number, additives for PCMs, storage unit dimension, heat exchanger surface enhancement, and sensible heating and sub-cooling, etc. The main perspectives and directions including heat transfer mechanism and optimized multiple PCM melting point are provided to enable further research.
This paper presents the evaluation of the performance of a ground source heat pump system monitored plant providing heating/cooling to an office building located in the Universitat Politècnica de València in Spain. The system was designed using GLHEPRO software and it has been monitored since 2005. Once a ground source heat pump has been designed, it is important to analyze its performance along the years after its construction and check whether the design was appropriate and the simulation predictions were consistent with real experimental measurements. This paper first presents the impact of the GSHP system in the ground thermal response. The simulations obtained in GLHEPRO software will be analyzed and compared to experimental measurements. The second purpose of this work is to compare the performance simulation results of a complete ground source heat pump system model built in TRNSYS, with the experimental measurements which have been registered and collected for one cooling day. Numerical predictions and experimental results are compared and discussed.
A quasi-3D model is used as a response model to generate normalized transfer functions of a borehole heat exchanger model. A solution is achieved by convolving in the spectral domain the transfer function of a given node with an input function describing the temperature change of the fluid over time. To demonstrate the accuracy and validity of the method, three comparison scenarios are studied. These scenarios compare the temperatures of a reference numerical model, the temperatures measured in the scope of a laboratory experiment, and during a field thermal response test. It is shown that the combined use of a spectral method and response model provides, in a few seconds, a temperature solution whose error is below or comparable to the measurement's uncertainty.
An improvement to the thermal resistance capacity model (TRCM) used to model borehole heat exchangers is presented. Here, the original model is extended to integrate the thermal capacities of the heat carrier fluid and pipe and to better account for the spacing between the pipes. Model results are compared to results provided by numerical models and show very good agreement. It is shown that the improved model brings a significant improvement for short times over the original model, allowing a rapid computation of the temperature response function at virtually any time and distance from a single borehole.
Buildings account for a significant proportion of the total energy and carbon emissions worldwide, and play an important role in formulating sustainable development strategies. There is a growing interest in ZEBs (zero energy buildings) in recent years. Several countries have adopted or considering establishing ZEBs as their future building energy targets to help alleviate the problems concerning the depletion of energy resources and the deterioration of the environment. Broadly speaking, ZEBs involve two design strategies – minimizing the need for energy use in buildings (especially for heating and cooling) through EEMs (energy-efficient measures) and adopting RETs (renewable energy and other technologies) to meet the remaining energy needs. This paper reviews the works related to these two strategies. EEMs include building envelopes, internal conditions, and building services systems; RETs cover photovoltaic/building-integrated photovoltaic, wind turbines, solar thermal (solar water heaters), heat pumps, and district heating and cooling. Issues pertaining to sustainable development implications and further research work required are also highlighted. These include life-cycle cost and environmental impacts, climate change and social policy issues.
In this paper an improvement of the model CaRM (CApacity Resistance Model) is presented to consider the borehole thermal capacitance, both of the filling material of the borehole and of the heat carrier fluid inside the ground heat exchanger. Several models, numerical and analytical, are available in literature for short time step analyses of ground-coupled heat pump systems. According to the modelling for the surrounding ground, the new approach for the inside of the borehole is based on electrical analogy. In this study the double U-tube ground heat exchanger is analyzed. The new model has been validated by means of a commercial software based on the finite elements method as well as measurements of ground response test, using a suitable plant system. In this last comparison, the contribution of the thermal capacitance of the circulating fluid is investigated, since it is frequently neglected in short time step simulations. In both cases, there is agreement between the CaRM results and data from numerical simulations and measurements as well.
Ground source heat pump systems often use vertical boreholes to exchange heat with the ground. Two areas of active research are the development of models to predict the thermal performance of vertical boreholes and improved procedures for analysis of in situ thermal conductivity tests, commonly known as thermal response tests (TRT). Both the models and analysis procedures ultimately need to be validated by comparing them to actual borehole data sets. This paper describes reference data sets for researchers to test their borehole models. The data sets are from a large laboratory “sandbox” containing a borehole with a U-tube. The tests are made under more controlled conditions than can be obtained in field tests. Thermal response tests on the borehole include temperature measurements on the borehole wall and within the surrounding soil, which are not usually available in field tests. The test data provide independent values of soil thermal conductivity and borehole thermal resistance for verifying borehole models and TRT analysis procedures. As an illustration, several borehole models are compared with one of the thermal response tests.
This paper presents the development and application of a three-dimensional (3D) numerical simulation model for U-tube borehole heat exchangers (BHEs). The proposed model includes the thermal capacities of the borehole components, viz., the fluid inside the tubes, as well as the grouting material, making it possible to consider the transient effects of heat and mass transports inside the borehole. In this approach, the use of simplified thermal resistance and capacity models (TRCMs) provides accurate results while substantially reducing the number of nodes and the computation time compared with fully discretized computations such as finite element (FE) models. The model is compared with a fully discretized FE model which serves as a reference. Furthermore, the model is used to evaluate thermal response test (TRT) data by the parameter estimation technique. Comparison of the model results with the results of an analytical model based on the line-source theory further establishes the advantage of the developed 3D transient model, as the test duration can be shortened and results are more accurate.
Residential Ground Coupled Heat Pump systems are usually characterised by an ON/OFF behaviour of the heat pump with typical cycling frequencies of 1—4 cycles per hour. The ground loop fluid pump has the same ON/OFF behaviour and the borehole heat exchanger operates either in full flow or no flow conditions. Typical hourly simulations of GCHP systems use steady-state models for the heat pump and the borehole fluid (transient models being used for buildings and heat transfer in the ground). This paper reviews the models used in typical hourly simulations as well as transient models that are available and compares the results obtained using the two classes of models within the TRNSYS simulation environment. Both the long-term energy performance and the optimum system design are compared. It is shown that using steady-state models leads to an overestimation of the energy use that ranges from a few percents with oversized borehole heat exchangers to 75% for undersized exchangers. A simple Life Cycle Cost analysis shows that using steady-state models can lead to selecting a very different design than the one that would have been selected using dynamic models.
Vertical U-tube ground heat exchangers are a key component in geothermal energy utilization systems like ground source heat pumps (GSHPs). This paper presents a three-dimensional unstructured finite volume model for them. The model uses Delaunay triangulation method to mesh the cross-section domain of the borefield (borehole field), and consequently may intactly retain the geometric structure in the borehole. To further improve the computational accuracy, the soil is divided into many layers in the vertical direction in order to account for the effect of changing fluid temperature with depth on the thermal process in the borefield. The inlet temperature of the ground heat exchanger (GHE) is used as a boundary condition, and the inside and outside surfaces of the U-tube pipes are treated as the conjugated interfaces in the domain. Thus, the conjugate thermal processes between the fluid in the pipes and the soil around it and between the two pipe legs may be accounted fully. A comparison of the model predictions and experimental data shows that the model has good prediction accuracy.
Many models, either numerical or analytical, have been proposed to analyse the thermal response of vertical heat exchangers that are used in ground coupled heat pump systems (GCHP). In both approaches, most of the models are valid after few hours of operation since they neglect the heat capacity of the borehole. This is valid for design purposes, where the time of interest is in the order of months and years. Recently, the short time response of vertical boreholes became a subject of interest. In this paper, we present a new analytical approach to treat this problem. It solves the exact solution for concentric cylinders and is a good approximation for the familiar U-tube configuration.
The short-term behavior of ground-coupled heat pump systems is important for design of ground loop heat exchangers, energy analysis of ground source heat pump systems, and design of hybrid ground source systems. This paper describes the development of short time-step temperature response factors for vertical ground loop heat exchangers as used in ground-coupled heat pump systems. The short time-step response factors allow for a direct evaluation of system energy consumption and electrical demand in hourly or shorter time intervals. The development of the temperature response factors is based on an analytically validated, transient two-dimensional implicit finite volume model designed for the simulation of heat transfer over a vertical U-tube ground heat exchanger. The short time-step response factors are implemented as part of a component model for TRNSYS and an example application is provided based on an actual building.
DOE-2 (DOE2 90) includes several correlation curves that predict the energy use of systems underpart load conditions. DOE-2 simulates systems on an hour-by-hour basis, so the correlations are intended to predict part load energy use (and efficiency) as a function of the part load ratio (PLR) for each hour, where PLR = Hourly Load/Available Capacity. Generally residential and small commercial HVAC equipment meets the load at off-design conditions by cycling on and off. Therefore, the part load correlations must predict the degradation due to this on and off operation over an hourly interval.
The detailed design and energy analysis of ground source heat pump systems requires the ability to predict the short-term behavior of borehole heat exchangers (BHE). The application of fully discretized models leads to extensive computation times and a substantial effort in terms of pre-processing work. On the contrary, analytical models offer simple, parameter input-based modeling and short computation times, but they usually disregard the transient effects of heat and mass transport in the borehole and hence are not suitable for the prediction of the short-time behavior. In order to combine the advantages of both types of models, the authors developed two-dimensional thermal resistance and capacity models for different types of BHE. These models take the capacity of the grouting material with one capacity per tube into account and, therefore, the range of validity is extended to shorter times. The correct consideration of all thermal resistances between the fluid in the pipes, the grout capacities and the borehole wall is important because of the significant influence on the validity of the models. With the developed models, the modeling work and the computation time can be significantly reduced compared with fully discretized computations while precise results are still achieved. The validation of the suggested models against fully discretized FEM models shows a very good agreement. Copyright © 2010 John Wiley & Sons, Ltd.
Thesis (M. S.)--Oklahoma State University, 2005. Includes bibliographical references (p.146-148). Vita. The full text of the thesis is available as an Adobe Acrobat pdf.file (xv, 172 p.); Adobe Acrobat Reader required to view the file.
Handbook of integrated and sustainable buildings equipment and systems, volume I: energy systems
- G Li
- Y Hwang
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