The performance of a magnetocaloric heat pump (MCHP) consisting of active magnetocaloric regenerators (AMR) of 12 layers of MnFePSi magnetocaloric materials (MCM) with a linear distribution of Curie temperatures was investigated using a 1D numerical model. The model predicted the heating power and coefficient of performance (COP) of the AMR for a fixed temperature span of 27 K, between 281 K and 308 K, and variable flow rate and AMR cycle frequency. A maximum applied magnetic field strength of 1.4 T was used. A well-insulated house with a maximum heating power demand of 3 kW (under quasi steady state conditions) was considered. Ambient temperature in The Netherlands was taken as a reference for the estimation of the seasonal heating power demand. Without optimizing the design of the AMR, the model predicts a maximum single-AMR heating power equal to 43.5 W when the AMR operates at 3 Hz and 3 L min^-1, and a maximum COP equal to 5.8 when it operates at 1.5 Hz and 1 L min^-1 Considering the maximum heating power of a single AMR, approximately 69 AMRs are needed to provide the design heating power demand of the house. It was found that it is possible to achieve an AMR seasonal COP of 5.6 by continuously adjusting the flow rate and frequency of operation of the MCHP along with the ON/OFF switching of some groups of AMRs in order to adjust the heating power of the MCHP to the heating power demand of the house.

@en

The development of affordable magnetocaloric materials (MCM) with a giant magnetocaloric effect (MCE) has brought magnetocaloric heat pumps a step closer to commercialization. The narrow temperature range in which these materials exhibit a large MCE demands the use of several materials with Curie temperatures covering the temperature span of the heat pump in a so-called layered active magnetocaloric regenerator (AMR). How to place these materials in the AMR in terms of distribution of Curie temperatures and thickness of each layer is still a topic of study. In this research we used a one dimensional numerical model to unveil potential benefits of either using a distribution of Curie temperatures that follows a sigmoidal shape or using thicker layers at the cold and hot ends of the AMR along with a linear distribution of Curie temperatures. We found that these AMRs are less sensitive to changes in the hot and cold reservoir temperatures compared to an AMR that uses just a linear distribution of Curie temperatures with uniform layer length, but only the one with thicker ends produces similar heating capacities and second law efficiencies. The heating capacity of the simulated AMR with a sigmoidal distribution of Curie temperatures varies only 5.6 % in a high utilization scenario, flow rate 37.5 g/s and a frequency of 0.75 Hz, when the hot side temperature changes from 308 K to 312 K and the temperature span is 18 K while the corresponding change is 8.7 % for the AMR with thicker end layers, and 37.9 % for the one with a linear distribution of Curie temperatures. For the considered geometry and operating conditions, the maximum heating capacities with temperature span 27 K in the high utilization scenario are 28.6 W, 23.0 W, and 28.5 W, whereas the corresponding second law efficiencies are 33.2%, 27.3 %, and 32.7% for the AMRs with linear distribution of Curie temperatures, sigmoid distribution, and linear distribution with thicker ends respectively.

@en

High concentration NH₃/H₂O is suitable for Kalina cycles used for the recovery of low grade heat. Plate heat exchangers (PHEs) are compact and reduce the charge of working fluid. This paper investigates the condensation of NH₃/H₂O with NH₃ mass concentrations of 80%-96%. The vapor and liquid concentrations are close to equilibrium state, which are different from normal absorbers. The apparent heat transfer coefficients (HTCs) and frictional pressure drop are presented, covering the mass fluxes of 32–86 kgm⁻²s⁻¹, the averaged vapor qualities of 0.08–0.65 and the saturated pressure of 610 to 780 kPa. Larger mass fluxes noticeably increase the apparent HTCs and frictional pressure drop. At the mass concentrations of 96%, 91% and 88%, higher vapor qualities increase the apparent HTCs for large mass fluxes. The apparent HTCs decrease slightly with vapor qualities for 80% mass concentration. The experimental results are compared with those of pure NH₃. The flow patterns of high concentration NH₃/H₂O are considered as full film flow and partial film flow, which are the same as for NH₃. The mass transfer resistance deteriorates the heat transfer especially for partial film flow, which happens at small liquid mass fluxes. The mass transfer resistance has negligible influences on frictional pressure drop.

@en

Ocean Thermal Energy Conversion (OTEC) produces electricity using the temperature difference between warm surface and cold deep-sea water. OTEC systems in literature only limitedly consider seasonal seawater temperature
variations and thus might not be adequately sized for off-design conditions. This potentially leads to techno-economically sub-optimal design choices. This paper sheds light on which design approach yields the most economically feasible OTEC system considering off-design conditions with 19 years of seawater temperature data in 3-h time steps. We find that systems sized for worst-case thermal resources yield the highest and steadiest electricity production. If seawater temperature variations are moderate, these systems also perform best economically in terms of Levelized Cost of Electricity (LCOE). We demonstrate our model for a 136 MWgross plant in Ende, Indonesia, with an LCOE of 15.12 US¢(2021)/kWh against a local electricity tariff of 15.77 US¢(2021)/kWh. The model is validated for different cost assumptions, system sizes, and temperature profiles to be useful globally. We give recommendations to curb costs and to move large-scale OTEC closer to today’s state of the art,
e.g. by using multiple smaller seawater pipes instead of few large pipes. The model is useful to prove OTEC’s global economic feasibility and to promote the technology’s commercialisation.@en

This study optimizes the district heating network side of a high temperature community heating system powered by decentralized solar collectors and seasonal thermal energy storage (STES). Six network configurations are considered which have the potential to improve system performance compared to a base scenario. The base scenario consists of a 2-line network with a fixed supply temperature where the decentralised solar collectors feed in over the heating network. All alternative configurations aim to improve system performance by lowering the temperature of consumed and/or produced heat. Lowering the temperature in the heating network reduces heat losses and decreases heat pump utilization. Lowering the operational temperature of the solar collectors increases their efficiency. The strategies explored by the different configurations include variable supply temperatures, a 4-line network (where the solar collectors do not feed into the heating network), and ways to mitigate temperature constraints imposed by domestic hot water production regulations. In the neighbourhood “”Karwijhof” of Nagele, 24 consumers will make the switch to a solar+storage district heating system. In order to assess their performance, all configurations and the base scenario are modelled in Matlab/Simulink. The system performance is measured in terms of levelised cost of heat (LCOH) and seasonal coefficient of performance (SCOP). They are compared to a scenario where the dwellings are fitted with individual high temperature air to water heat pumps. Making the supply temperature variable (dependent on the ambient temperature) reduces pipeline thermal losses and reduces heat pump utilization. The transition from a 2-line network to a 4-line network where the solar collectors are separately connected to the buffer was found to significantly increase solar collector efficiency. The combination of these two measures reduces the LCOH by 4.5 %. Slightly oversizing the buffer volume and solar area significantly increases the SCOP with small impact on LCOH. When comparing the improved community solar heating system with a scenario where every house is heated with an individual heat pump instead, it is found that the community solar system achieve a 15.7 % lower LCOH while having a SCOP of 4.4 compared to just 2.75 for the heat pump scenario.@en

This study assesses the role of (seasonal) thermal energy storage in the next generation renewables based central heating systems for the built environment in the Netherlands. Specifically, the neighbourhood "Karwijhof" in the city Nagele which is transitioning to a collective renewable district heating network incorporating 24 users. The study focus on the technology for storing thermal energy and two different heat collection technologies. The storage of heat is done using an underground seasonal thermal energy storage (USTES), in this case an underground sensible heat storage tank using water as storage medium. The system relies on a small scale district heating network (DHN) for the distribution of heat. For this research two heat collection technologies are considered both incorporating the USTES as main system component. The first system relies on heat collection by solar thermal collectors, the second on an air-water heat pump. Both systems are modelled in Matlab-Simulink making use of KNMI weather data. Different system sizes are evaluated. The investigated components include: volume of the USTES, surface area of the solar thermal collectors, and air-water heat pump capacity. Key performance indicators include the levelised cost of heat (LCOH) and the seasonal COP of the system which gives an indication on the autonomy of the system. To increase the autonomy of the systems a photo-voltaic (PV) array is considered for both systems to offset the electricity use. However, the systems are allowed to exchange electricity with the grid translating into the goal of "zero on the meter" autonomy. The results show that both systems can ensure heat throughout the year for the users considered during this study. However, systems cannot compete with traditional natural gas heating systems based on the LCOH. This is partly due to the high cost of the district heating network. The systems including a PV array show a LCOH that can compete with the traditional natural gas HR-boiler but are constraint by the rooftop area available during this study leading to a non-competitive LCOH. When considering the environmental benefits, the systems are already competitive to the traditional natural gas heating systems.@en

Within the agriculture sector, drying of food and flowering products is a required production step. Most companies use hot air dryers where the heat is obtained by combustion of natural gas. This paper proposes a heat pump assisted drying system as an alternative. Flower bulb carrying boxes containing just harvested products have been approximated as packed beds filled with spherical products. A heat pump is proposed which includes a cross flow heat exchanger and allows for recirculation of the drying air while maintaining its operating temperature and humidity at optimal conditions to guarantee the required drying rate while preventing damage of the product. Simulink is used to optimize the operating conditions of the system guaranteeing both the product quality and a low energy consumption. The heat pump has been sized to dry 10 flower bulb carrying boxes of about 800 kg each in 24 hours. The performance of the heat pump (170 kWh/day renewable electricity) is compared to the performance of conventional fossil fuel based drying systems (350 kWh/day fossil fuel). Understanding heat and mass transfer mechanisms during drying allows optimization of the heat pump design.@en

Buildings consume 40% of the total global energy and contribute to over 30% of all CO2 emissions. Space heating forms a significant part of energy consumption in buildings. A possible solution is to recover waste heat from sewage and to upgrade its exergy using a heat pump. Polymers reduce cost and energy required for manufacturing the heat exchangers. The impact of sewage flow and temperature, heat exchanger dimensions and thermal enhancement of polymers on heat recovery is studied using a Matlab model. The model integrates a sewage heat exchanger with a heat pump and optimizes heat recovery from sewage. A heat pump is used to obtain hot water at 55°C for space heating. Heat recovered from sewage and overall COP are quantified. In case of HPDE with graphite filler, increase in filler content from 0% to 30% increases thermal conductivity from 0.46 to 1.89 Wm-1K-1 increasing the heat recovery with 34%. The heat transfer coefficient becomes twice as large when polymer with no filler is enhanced with 30% filler content. Large heat exchange area and variability of sewage level limit the recovered heat.@en

High concentration NH3/H2O is suitable for Kalina cycles used for the recovery of low grade heat. Plate heat exchangers (PHEs) are compact and reduce the charge of working fluid. This work investigates the absorption of NH3/H2O in a PHE, which has a weight concentration of 96%. Being different from normal absorption systems, the concentration difference between vapor and liquid is small, and apparent heat transfer coefficients (HTCs) are used to interpret the phase change process, which assumes that the vapor and liquid are in equilibrium state. The apparent HTCs and frictional pressure drop of NH3/H2O are presented and are compared with those of NH3. The mass transfer resistance has noticeable influences on heat transfer depending on the flow patterns, while the influence on frictional pressure drop is minor.@en

A comparative study for a representative case district in the Netherlands with around 2500 dwellings has been executed with the purpose of identifying the best solution for a Low Temperature District Heating (LTDH) with Low Temperature Geothermal Heat (LTGH) as the main heat source. The district is presently connected to the Dutch gas network. Its heat demand and building typologies are used as departing points. The comparison is based on 3 key performances indicators (KPI’s): CO2 emissions, levelized costs of energy (LCOE) and peak electricity use. The following LTDH designs have been considered: Central heat pump and collective peak supply at 70 ⁰C / 50 ⁰C; Central heat pump and decentral peak supply at 70 ⁰C / 50 ⁰C; Decentralized heat pumps using 30 ⁰C supply temperature. Individual air source heat pumps for space heating and electric boilers for domestic hot water purposes are taken as reference. The LTDH concept with decentralized heat pumps saves 19.5 %, 16.8 % and 36 % on the CO2 emission, LCOE, and electricity use in peak load hours compared to the reference concept.@en

Seasonal thermal energy storage (STES) systems in combination with heat pumps can significantly reduce the impact of peak loads in large scale district heating systems and allow for the application of renewable heat sources in these networks. This paper investigates technologies with the highest potential for implementation in large scale district heating networks and identifies high temperature aquifer thermal energy storage (HT-ATES) as the most suitable technology. The HT-ATES has been applied at the primary side of the network (90 to 110 oC) and at the secondary side (72 to 92 oC). Suitable locations for the STES have then been identified. A control volume approach is used in Matlab to predict heat transfer and pressure drop in the HT-ATES wells considering porosity, permeability, grain size and geometry of the aquifer. The temperature distribution and extraction temperature are predicted as function of time. The yearly heating demand of a network has been used in combination with different operating modes to identify the required size of the HT-ATES. The simulations indicate that it will take 5 years to reach steady temperature supply.@en

Heat pumps can efficiently upgrade waste heat from the industry and in that way reduce emissions. One of the main reasons why heat pumps are not applied to a greater extent in industry are large payback periods. Compression–resorption heat pumps (CRHP) enhanced by wet compression are considered a very promising option that can have higher coefficient of performance compared to traditional technologies when the heat source and/or sink have a large temperature glide. In this study the thermodynamic and economic performance of two potential industrial cases are examined for CRHP operating with NH₃–H₂O and NH₃–CO₂–H₂O. A detailed thermodynamic model of the compressor is used to evaluate the isentropic efficiency for each case. The results are used to calculate the simple payback period, when a boiler is replaced by a CRHP, as a function of the predicted gas and electricity prices in the Netherlands from 2020 to 2035. The results indicate that adding CO₂ to the NH₃–H₂O mixture increases the cycle COP when the temperature glide of the heat sink is 40 K while the opposite occurs when the glide is 80 K. The highest COPs and lowest payback times are obtained when the outlet vapor quality is around 0.50 for both the binary and ternary mixtures. Larger glides require higher outlet qualities. However, it is clear that even for high temperature glides the payback period can be within acceptable limits, especially if the cost of CO₂ emissions is taken into account.

@en

This paper investigates NH₃ condensation in a plate heat exchanger by visualizing the flow patterns and measuring heat transfer coefficients and frictional pressure drop. Visualization experiments were conducted between 20 and 100 kgm⁻²s⁻¹. Full film flow takes place at large mass fluxes and intermediate mass fluxes of low vapor qualities, while partial film flow occurs at small mass fluxes and intermediate mass fluxes at high vapor qualities. The heat transfer and frictional pressure drop experiments cover the mass fluxes of 21~78 kgm⁻²s⁻¹, the averaged vapor qualities of 0.05~0.65 and the saturated pressure of 630 to 930 kPa. Vapor qualities have significant influences on heat transfer and frictional pressure drop. In the tested ranges, the effect of mass fluxes is noticeable on frictional pressure drop, but is moderate on heat transfer. The impact of saturated pressure is small. The heat transfer reflects the change of flow patterns. The frictional pressure drop shows the characteristics of separated flow.

@en

Heat pumps can drastically reduce energy requirements in industry. Operating a compression resorption heat pump with an NH₃-CO₂-H₂O mixture has been identified as a promising option that can have an increased performance compared to only NH₃-H₂O. In this paper an important process of the heat pump cycle is investigated: The absorption process. A mini-channel heat exchanger with 116 tubes of inside diameter of 0.5 mm is used for this purpose. For the NH₃-H₂O experiments overall heat transfer coefficients of 2.7–6 kW/(m²K) were reached for mixture mass flows of 0.71–2.5 kg/h. For the NH₃-CO₂-H₂O mixture pumping instabilities limited the operating range which resulted in higher pressures and higher mixture mass flows compared to NH₃-H₂O. The overall heat transfer coefficients were lower in the case of the added CO₂, with the maximum of 3 kW/(m²K) corresponding to a mixture mass flow of 4.2 kg/h. However, an increase in heat transfer of approximately 5% was reached with the added CO₂ which is beneficial for heat pump applications. Additionally, limited research has been conducted on absorption in upward versus downward flow and, therefore, these two configurations have also been tested in the mini-channel heat exchanger. Even though the pumping instabilities vanished with absorption in upward flow it was confirmed that absorption in downward flow with the mixture on the tube side is the most beneficial configuration for absorption of ammonia in NH₃-CO₂-H₂O or NH₃-H₂O in a mini-channel heat exchanger. The performance increased by approximately 10% with absorption in downward flow.

@en

This paper develops predicting models for NH₃ condensation in plate heat exchangers based on the experiments of flow patterns, heat transfer coefficients and frictional pressure drop previously reported by the authors. The aim is to provide design methods of compact plate condensers used in NH₃ systems, which are not available in open literature. The experimental data are firstly compared with selected correlations, showing a poor agreement. A heat transfer model is developed based on flow patterns, which represents the transition from convective condensation to gravity-controlled condensation. The physical interpretation of the two-phase multiplier approach and the deviation from Nusselt's theory are discussed. A transition criterion of condensation mechanisms is proposed based on the wetting characteristics. Since the flow patterns indicate separated flow, the Lockhart and Martinelli model is selected and is modified to predict the frictional pressure drop. The model is the sum of the liquid pressure drop, vapor pressure drop and interface pressure drop. The contributions of vapor pressure drop and interface pressure drop are discussed and quantified. The proposed heat transfer and frictional pressure drop models show good predictive performances. NH₃ flow has large two-phase slip because of the large density ratio. Plate heat exchangers have corrugated channels and tend to break up the liquid film. The models identify the distinct flow characteristics based on flow patterns.

@en

Most plate heat exchangers (PHEs) have hydraulic diameters in the range of 2∼5 mm and show characteristics of both macro-channels and micro-channels. Both gravity and surface tension have non-negligible influences and determine the heat transfer and frictional pressure drop.

This paper investigates NH3 condensation in a PHE with a hydraulic diameter of 2.99 mm. The large surface tension of NH3 enhances the micro-channel characteristics. The heat transfer coefficients (HTCs) are compared with homogeneous and separated models, respectively. Both models have been previously compared with the experimental data of HFCs, hydrocarbons and HFOs. The prediction for NH3 is generally good since the deviations are small, while the sensitivity to mass fluxes and vapor qualities cannot be estimated properly. The data of frictional pressure drop are predicted by a correlation of two-phase Fanning friction factor, which is based on homogeneous flow and includes the influences of mass fluxes, vapor qualities, hydraulic diameters, chevron angles, etc. The fluid properties of NH3 are significantly different between liquid and vapor phases, and the averaged density derived from homogeneous flow is under-estimated. The prediction is improved by calculating the averaged density from the void fraction models of separated flow.
@en

Solar-based poly-generation systems with thermally driven sorption heat pump / chillers use the condensation heat of ORC to cover heating and cooling loads of buildings. In the present study the performance of such systems when used in buildings in the Netherlands and Greece is investigated taking both exergy efficiency and economic aspects into account. The alternative of using PV-panels and electrically driven vapour compression heat pumps / chillers is also investigated. The performance of the systems is compared to the performance of conventional heating and cooling systems. Higher exergy efficiencies can be attained with the ORC sorption systems only for unacceptably long payback times. Optimum systems will have low exergy efficiency (8 %) compared to the exergy efficiency of conventional systems (16 %) and still long payback times (longer than 19 years). PV - panel vapour compression heat pump / chiller systems have generally acceptable payback times smaller than 10 years.

@en

Compression-resorption heat pumps (CRHP) utilizing wet compression are a very promising option to upgrade waste heat from industry. CRHPs have the potential to have higher coefficient of performance (COP) than the traditionally used vapour-compression heat pumps (VCHP). However, commercial solutions utilizing wet compression are not available yet. Also, wet compression is a feasible option only if the efficiency of the compressor is sufficiently high, 0.7 or higher, as identified by several authors. In this study, we develop and validate a model of a twin screw compressor that is suitable for wet compression. The model is adapted to calculate the entropy production generation in order to identify where the major irreversibilities are located in the compressor. The effects of clearance size, rotational speed, ammonia concentrations, compressor inlet vapor quality as well as under- and over compression are analysed. The results show that the clearance size and the rotational speed have the largest effects on the entropy production. Additionally, increased ammonia concentration and decreased vapor quality lead to decreased losses. The results indicate that it should be feasible to reach the targeted performance if the clearances size is limited to 50 μm, the rotational speed maintained above 10,000 rpm, the ammonia concentration kept in the range of 30–40 wt.%, and the inlet vapor quality in the range 0.5–0.7.

@en

The corrugation channels of plate heat exchangers enhance the heat transfer and complicate the prediction of heat transfer coefficients and frictional pressure drop. This paper reviews the heat transfer and frictional pressure drop correlations for condensation in plate heat exchangers, and classifies the correlations into basic forms. An experimental database is developed including the data of HFCs, hydrocarbons, HFOs and CO ₂ . The mass fluxes are in the range of 2–150 kg·m ⁻² ·s ⁻¹ . The chevron angles and hydraulic diameters are distributed in 25.7°–70° and 3.23–8.08 mm. The saturated temperatures are −34.4 to 72.1 °C, while the reduced pressures are from 0.03 to 0.49. Eight heat transfer correlations are assessed with the database. The correlation of Longo et al. [48] predicts the experimental data best, while the correlation of Kuo et al. [16] shows the second best performance. Six frictional pressure drop correlations are compared with the database. The prediction of frictional pressure drop is relatively poor, and a new correlation is developed using multi-variable regression analysis with non-dimensional numbers. This new correlation predicts 87.5% of the experimental data within ±50%.

@en

The heat transfer coefficients (HTCs) of NH₃ are higher than of HFCs and hydrocarbons during the condensation within plate heat exchangers (PHEs) mainly due to its favorable transport properties. NH₃/H₂O has a large temperature glide, and its heat transfer is dependent on mass transfer. Few models have been specifically developed for NH3 and NH₃/H₂O condensation in PHEs. This paper presents heat transfer and pressure drop experiments for partial condensation. The calculation method for condensation HTCs and frictional pressure drop is introduced. The working fluids are pure NH₃ and NH₃/H₂O with a weight concentration of 96%. For a mass flux of 62 kgm^-2s^-1, the HTCs of NH3 increase from 10 to 20 kWm^-2K^-1 with vapor quality. The apparent HTCs of NH₃/H₂O are significantly lower than NH₃ at high vapor qualities because of mass transfer resistance. Condensation pressure level has a slight influence on HTCs and frictional pressure drop.

@en