This paper reports a study on how hourly temperature variations of different heat sources influence the seasonal coefficient of performance (SCOP) of heat pumps (HPs) when supplying district heating. ...The considered heat sources were: groundwater, seawater, air and a combination of the three. The system included HPs, an electric peak load boiler and short-term storage. Linear programming was used to minimize annual electricity consumption of the system. This process also determined the optimum capacities of the HPs using different heat sources. The study was based on data for the area of Copenhagen, Denmark.
The results showed that the SCOP of seawater and air HPs, considering heat demand variations, was 11% and 15% lower, respectively, than their arithmetic mean performances. For a combination of heat sources, the optimum proportions of HP capacities were: 63%, 14% and 23% for the groundwater, seawater and air HP, respectively. The SCOP of such system was found to be 3%, 6% and 11% greater than the SCOP of a system using the heat sources individually. The results indicate that a maximum system performance may be achieved for HPs based on a combination of different heat sources.
•The seasonal COP of heat pumps (HPs) is lower than the arithmetic mean COP.•HPs using different heat sources can perform better than HPs using a single one.•Hourly COPs of HPs are preferable, if different heat sources are used.•The preferred heat source to use by HPs varies during the year.•Heat source proportion, HP capacities, climate and building type affect the COP.
Often, simple estimates of the coefficient of performance (COP) for heat pumps (HPs) and chillers are used. Depending on the purpose, this may not be sufficient. There are more accurate methods for ...determining COP, but they may not be used due to complexity, nonlinearity, or limited application of advanced COP estimation methods. Here, we present a new COP approximation method suitable for HPs and chillers. It is based on linear relationships and was developed from a thermodynamic two-stage HP model for design and off-design operation using ammonia as a refrigerant. This approximation method was then applied to a case study investigating the potential of district heating supplied by HPs in Tallinn, Estonia. Groundwater, sewage water, seawater, river water, lake water, and a district cooling return line were explored as potential heat sources. The results show a deviation in COP of less than 1.5% compared to the thermodynamic model. Annual calculations show the applicability of the COP approximation method for calculating hourly COPs at different heat source and heat sink temperatures, as well as changing heat loads, seasonal COP, heat demand ratio, and hourly Lorenz efficiency.
•New method of linear approximation of COP for heat pumps and chillers.•COP approximation method suitable for annual off-design operation.•Ease of use, very accurate and suitable for linear programming.•Applied to district heating and six various heat sources.•COP deviations under 1.5% compared to results of thermodynamic two-stage HP model.
This paper compares estimation of the coefficient of performance (COP) of a large-scale heat pump (HP) for district heating based on four methods to the COP obtained using a detailed thermodynamic HP ...model. Four heat sources and varying district heating supply temperatures were considered. The COP estimation methods are based on constant COP, Lorenz efficiency, exergy efficiency and a method presented by Jensen et al. (2018). They were implemented in an energy planning tool and further analysed. The planning tool was used to assess HP implementation in a new district in Copenhagen, Denmark. The change in seasonal COP of the HPs, the economic results and optimal HP capacities were compared.
The results show that the Jensen et al. (2018) method provides good approximations and that the planning tool identifies a similar solution compared to the use of the thermodynamic HP model. Assuming a constant Lorenz efficiency, exergy efficiency or COP over the year resulted in large deviations in COP, especially for operations very different from the design conditions. Consequently, other heat sources were found for the most economical solution. The accuracy of the three methods decreased when the initial assumptions of constant COP and efficiencies differed from the ones at design conditions.
Large-scale heat pumps (HPs) and refrigeration plants are essential technologies to decarbonise the heating and cooling sector. District heating and cooling (DHC) can be supplied with low carbon ...footprint, if power generated from renewable energy sources is used. The simultaneous supply of DHC is often not considered in energy planning, nor the characteristics of the heat source and sink. Simplified approaches may not reveal the true potential of HPs and chillers. In this paper, different heat sources and sinks and their characteristics were considered for the simultaneous supply of DHC based on large-scale HPs and refrigeration plants. An optimization model was developed based on mixed-integer linear programming. The model is able to identify ideal production and storage capacities, heat sources and sinks based on realistic hourly operation profiles. By doing so, it is possible to identify the most economical or sustainable supply of DHC using electricity. The optimization model was applied to the Nordhavn area, a new development district of Copenhagen, Denmark. The results show that a combination of different heat sources and sinks is ideal for the case study. A HP that uses the district cooling network as a heat source to supply DHC was shown to be very efficient and economical. Groundwater and sewage water HPs were proposed for an economical supply of district heating. The Pareto frontier showed that a large reduction in annual CO2 emissions is possible for a relatively small increase in investments.
Large-scale heat pumps (HPs) are proposed as a technology to efficiently utilize intermittent wind power and other renewable sources. More than 25 large-scale HPs have been installed over the past ...decade to supply district heating (DH) in Denmark. A continuous increase is expected in the coming years. The HP projects differ in size, configuration, components and heat source. All these have an impact on the investment costs, which poses challenges for estimating costs, e.g. when planning new HP projects. For this paper, the investment costs of existing and planned electrically driven large-scale HPs were analyzed. All analyzed HPs use natural refrigerants and supply DH in Denmark. The total investment costs were divided into different categories to identify cost correlations for each of them depending on the heat source and HP capacity. The developed cost correlations were combined and verified by comparing the resulting correlations with the total investment costs of the considered HPs. Different intervals of the specific total investment costs for HPs depending on the heat source and HP capacity were derived. They identified the most and least expensive heat sources for HP capacities between 0.5 MW and 10 MW. It was shown that a considerable amount (~50 %) of the investment costs was placed on other parts than the HP itself.
This paper presents a derivation of design guidelines for plate heat exchangers used for evaporation of zeotropic mixtures in heat pumps. A mapping of combined heat exchanger and cycle calculations ...for different combinations of geometrical parameters and working fluids allowed estimating the trade-off between heat transfer area and pressure drops on the thermodynamic and economic performance indicators of the cycle. Compressor running costs constituted the largest cost share, and increased due to a steep decrease of the heat pump coefficient of performance at high refrigerant pressure drops. It was found that the pressure drop limit leading to infeasible designs was dependent on the working fluid, thereby making it impossible to define a guideline based on maximum allowable pressure drops. It was found that economically feasible designs could be obtained by correlating the vapour Reynolds number and the Bond number at the evaporator inlet as ReV−0.42Bd0.26≈0.040. The use of the proposed guideline was illustrated for the mixture Propane/Iso-Pentane (0.5/0.5), leading to evaporator designs with net present values deviating maximum −4.4% from the best value found in the mapping. The presented methodology can be applied in different scenarios to develop similar guidelines, thereby decreasing the cost of combined cycle and component optimizations.
•Maximum allowable pressure drop do not lead to optimal heat exchanger design.•Refrigerant pressure drop negatively impacts both COP and NPV of heat pumps.•Different optimal pressure drop are found depending on the working fluid.•A HEX design guideline was derived by correlating non-dimensional parameters.
Two new cold plates manufactured via metal 3D printing were experimentally investigated for thermal performance analysis in indirect liquid cooling operations; then they were compared to the ...traditional cold plates. Experiments were performed with different coolant inlet temperatures (15.7 °C and 24.5 °C) and ambient air velocities (0.5 m/s and 3 m/s) at tropical conditions; hereby, the impact of high dew point temperatures at tropics was also investigated. Body-centered cubic (BCC) and pillar elements were applied in the cooling cavity of the cold plates. The results showed that the target surface temperature in both BCC- and pillar-filled plate designs was maintained below the limits at the lower inlet temperature. However, at the higher inlet temperature, the temperature was only maintained below the limit when the ambient air velocity was 3 m/s. The convective heat transfer coefficient at the inlet temperature of 15.7 °C was found 1.5 and 2.5 times higher than the convective heat transfer coefficient value at the inlet temperature of 24.5 °C for the pillar- and BCC-filled plates, respectively. The performance evaluation criterion values were found in the range of 1.2 − 2.4, which depended on the operating conditions and were already higher than the referenced studies in the literature.
This study aims to characterize experimentally the heat transfer in micro-milled multi-microchannels copper heat sinks operating with flow boiling, in the attempt to contribute to the development of ...novel and high heat flux thermal management systems for power electronics. The working fluid was R-134a and the investigation was conducted for a nominal outlet saturation temperature of 30 ∘C. The microchannels were 1 cm long and covered a square footprint area of 1 cm2. Boiling curves starting at low vapor quality and average heat transfer coefficients were obtained for nominal channel mass fluxes from 250 kg/m2s to 1100 kg/m2s. The measurements were conducted by gradually increasing the power dissipation over a serpentine heater soldered at the bottom of the multi-microchannels, until a maximum heater temperature of 150 ∘C was reached. Infrared thermography was used for the heater temperature measurements, while high-speed imaging through a transparent top cover provided visual access over the entire length of the channels. The average heat transfer coefficient increased with the dissipated heat flux until a decrease dependent on hydrodynamic effects occurred, possibly due to incomplete wall wetting. Depending on the channel geometry, a peak value of 200 kW/m2K for the footprint heat transfer coefficient and a maximum dissipation of 620 W/cm2 at the footprint with a limit temperature of 150 ∘C could be obtained, showing the suitability of the investigated geometries in high heat flux cooling of power electronics. The experimental dataset was used to assess the prediction capability of selected literature correlations. The prediction method by Bertsch et al. gave the best agreement with a mean absolute percent error of 24.5%, resulting to be a good design tool for flow boiling in high aspect ratio multi-microchannels as considered in this study.
The present study investigated a new microchannel profile design encompassing condensate drainage slits for improved moisture removal with use of triangular shaped plain fins. Heat transfer and ...pressure drop correlations were developed using computational fluid dynamics (CFD) and defined in terms of Colburn j-factor and Fanning f-factor. The microchannels were square 2.00 × 2.00 mm and placed with 4.50 mm longitudinal tube pitch. The transverse tube pitch and the triangular fin pitch were varied from 9.00 to 21.00 mm and 2.50 to 10.00 mm, respectively. Frontal velocity ranged from 1.47 to 4.40 m·s−1. The chosen evaporator geometry corresponds to evaporators for industrial refrigeration systems with long frosting periods. Furthermore, the CFD simulations covered the complete thermal entrance and developed regions, and made it possible to extract virtually infinite longitudinal heat transfer and pressure drop characteristics. The developed Colburn j-factor and Fanning f-factor correlations are able to predict the numerical results with 3.41% and 3.95% deviation, respectively.
The present study investigates cold air recirculation in the evaporators of large-scale air-source heat pumps. A case study considered a 5 MW air-source heat pump producing heat for district heating. ...The heat pump comprises 20 horizontal evaporators, where each evaporator is equipped with eight fans. The evaporators were implemented in a CFD model, where the influence of the outdoor wind direction on the recirculation was investigated. Firstly, the air recirculation was analysed with no surrounding obstacles. Secondly, the surrounding building and the real ground topology was included in the CFD model, to analyse their influence on the air recirculation. The results show that recirculation occurs for all wind directions, due to the turbulent behaviour of the flow around the evaporators. The results also show that the presence of a building intensifies the recirculation when it is placed directly upstream of the evaporators due to the presence of vortices in the wake of the building. On the other hand, a ground depression helps to reduce the recirculation by having additional energy dissipation due to the sudden change in the ground direction.
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