► Numerical analysis on the performance of a thermal storages as free cooling system. ► Employing multiple PCMs to enhance heat transfer rate in thermal storages. ► Using an effective heat capacity ...method, the phase change parameters are determined. ► The effect of the slabs size and air channel thickness on COP is investigated.
As demand for refrigeration and air conditioning increased during the last decade, the opportunities have expanded for using thermal energy storage (TES) systems in an economically advantageous manner in place of conventional cooling plants. Many cool storage systems use phase change materials (PCMs) and achieve peak load shifting in buildings. This work presents numerical investigations of the performance enhancement of a free cooling system using a TES unit employing multiple PCMs. The TES unit is composed of a number of rectangular channels for the flowing heat transfer fluid, separated by PCM slabs. Using the effective heat capacity method, the melting and solidification of the PCM is solved. The forced convective heat transfer inside the channels is analyzed by solving the energy equation, which is coupled with the heat conduction equation in the container wall. The effect of design parameters such as PCM slab length, thickness and fluid passage gap on the storage performance is also investigated using an energy based optimization. The results show that a system which can guarantee comfort conditions for the climate of Tabriz, Iran has an optimum COP of 7.0. This could be achieved by a combination of CaCl2·6H2O with RT25 with the optimum air channel thickness of 3.2mm, length of 1.3m and PCM slab thickness of 10mm.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
•CO2/NH3 cascade refrigeration cycles with flash intercoolers are investigated.•Exergoeconomic factors of components are determined to assess their relative significances.•An environmental analysis ...is applied to determine the penalty cost of GHG emission.•The effects of operating parameters on COP, exergy efficiency and total cost rate are investigated.•An optimization is applied based on the maximum COP and the minimum total cost rate.
Exergoeconomic and environmental analyses are presented for two CO2/NH3 cascade refrigeration systems equipped with (1) two flash tanks and (2) a flash tank along with a flash intercooler with indirect subcooler. A comparative study is performed for the proposed systems, and optimal values of operating parameters of the system are determined that maximize the coefficient of performance (COP) and exergy efficiency and minimize the total annual cost. The operating parameters considered include condensing temperatures of NH3 in the condenser and CO2 in the cascade heat exchanger, the evaporating temperature of CO2 in the evaporator, the temperature difference in the cascade heat exchanger, the intermediate pressure of the flash tank in the CO2 low-temperature circuit, the mass flow rate ratio in the flash intercooler and the degree of superheating of the CO2 at the evaporator outlet. The total annual cost includes the capital, operating and maintenance costs and the penalty cost of GHG emission. The results show that, the total annual cost rate for system 1 is 11.2% and 11.9% lower than that for system 2 referring to thermodynamic and economic optimizations, respectively. For thermodynamic and cost optimal design condition the COP and exergy efficiency of both systems are almost the same. Finally, in order to obtain the best balance between exergy destruction cost and capital cost, the exergoeconomic factor is defined for each component of proposed systems, for cases in which the system operates at the best performance conditions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
Thermal energy represents a large part of the global energy usage and about 43% of this energy is used for industrial applications. Large amounts are lost via exhaust gases, liquid streams and ...cooling water while the share of low temperature waste heat is the largest.
Heat pumps upgrading waste heat to process heat and cooling and power cycles converting waste heat to electricity can make a strong impact in the related industries. The potential of several alternative technologies, either for the upgrading of low temperature waste heat such as compression-resorption, vapor compression and trans-critical heat pumps, or for the conversion of this waste heat by using organic Rankine, Kalina and trilateral cycle engines, are investigated with regards to energetic and economic performance by making use of thermodynamic models. This study focuses on temperature levels of 45–60 °C as at this temperature range large amounts of heat are rejected to the environment but also investigates the temperature levels for which power cycles become competitive. The heat pumps deliver 2.5–11 times more energy value than the power cycles in this low temperature range at equal waste heat input. Heat engines become competitive with heat pumps at waste heat temperatures at 100 °C and above.
•Application of heat pump technology for heating and cooling.•Compression resorption heat pumps operating with large glides approaching 100 K.•Compression-resorption heat pumps with wet compression.•Potential to convert Industrial waste heat to power or high grade heat.•Comparison between low temperature power cycles and heat pumps.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A state-of-the-art review is presented of the different technologies that are available to deliver refrigeration from solar energy. The review covers solar electric, solar thermal and some new ...emerging technologies. The solar thermal systems include thermo-mechanical, absorption, adsorption and desiccant solutions. A comparison is made between the different solutions both from the point of view of energy efficiency and economic feasibility. Solar electric and thermo-mechanical systems appear to be more expensive than thermal sorption systems. Absorption and adsorption are comparable in terms of performance but adsorption chillers are more expensive and bulkier than absorption chillers. The total cost of a single-effect LiBr–water absorption system is estimated to be the lowest.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Latent heat thermal storage (LHTS) is a technology that can help to reduce energy consumption for cooling applications, where the cold is stored in phase change materials (PCMs). Free cooling is a ...concept developed for air conditioning applications, in which coolness is collected from ambient air during night and released into the room during the hottest hours of the day. In this work, energy and exergy analyses are performed for a free cooling system using a LHTS unit employing multiple PCMs. The effects of inlet air temperature and air flow rate on the performance of the system are investigated. It is observed that the increase in exergy efficiency due to reducing inlet air temperature is more significant than effect from increasing the air flow rate during the charging process.
•The performance is investigated of LHTS free cooling system employing multiple PCMs.•The phase change process is predicted using the effective heat capacity method.•The effects of inlet air temperature and air flow rate on the exergy efficiency.•An optimization is performed based on the energy and exergy efficiencies.
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•NH3-CO2-H2O shows an increase in heat transfer of up to 5% compared to NH3- H2O.•The mixture should be on the tube side of mini-channel heat exchangers.•Vertical absorption in downward flow is ...significantly better than absorption in upward flow.•For NH3-CO2-H2O the absorber should be designed to prevent accumulation of CO2.
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Validated transient model of high temperature PCM thermal storage.•Charging and discharging of PCM function of solar availability and heat demand.•Selection of candidate high temperature ...PCM.•Quantification of additional energy savings of high temperature thermal storage.•Natural convection in liquid phase of PCM has high impact in model results.
Latent Heat Thermal Storage (LHTS) systems can be applied to minimize the discrepancy between energy supply and demand in solar applications. Combining this system with a double-effect H2O/LiBr absorption system could reduce the investment costs for cooling purposes. These systems require generation temperatures higher than 150°C. Phase Change Materials (PCMs) in the desired temperature range have previously not been thoroughly studied and the integration of an LHTS system into this absorption system has not previously been reported. In this paper a numerical investigation was carried out for a shell-and-tube LHTS system, combined with a solar driven H2O/LiBr double-effect absorption system. The numerical model of the LHTS system was coupled with a mathematical model of the absorption system, and the estimation of the cooling demand and the solar energy. The phase change process was numerically solved using the enthalpy method. Hydroquinone was identified as a suitable PCM. Results indicated that for this configuration of the LHTS system, natural convection cannot be neglected in the modelling of the solidification process. With a LHTS system of 12.55m3, it was possible to fulfil the considered 100kW peak cooling demand for the 2400m2 office building without external energy input. The presented work provides guidelines for the thermal performance and design optimization of a hydroquinone based LHTS system for a solar driven H2O/LiBr double-effect absorption system.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•It is crucial to account for the price of CO2 emissions when considering heat pumps.•The thermodynamic optimum is, in general, far off the economic one for CRHP.•The payback period for CRHP can be ...within 3 years for large range of conditions.
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The compressor efficiency makes CRHP performance condition dependent.•The efficiency is largely dependent on the vapor quality and NH3 concentration.•At lower vapor qualities liquid limits leakages ...and entropy production.•The locations of the ports are important to minimize losses.•For identical clearances the sealing line losses are the largest losses.
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•CO2 hydrate slurry has a high latent heat at temperatures in the range of 7–8°C.•Fluidized bed heat exchanger (FBHE) allows for continuous generation of hydrates.•Hydrate production rate in the FBHE ...can be predicted using a crystal growth model.•FBHE operation is possible up to 35wt% in open systems and 45wt% in closed systems.•Energy savings up to 45% are possible using day/night gradient and hydrate storage.
Latent heat thermal storage (LHTS) systems can be applied to minimize the discrepancy between energy supply and demand in cold applications. Combining this system with night time generation of CO2 hydrate slurry can significantly reduce the investment costs for cooling purposes. These systems require hydrate generation temperatures around 8°C and can, from an energetic point of view, contribute to significant energy savings. In this paper a numerical and experimental investigation is carried out for a shell-and-tube fluidized bed heat exchanger, combined with a hydrate slurry storage vessel. When required, heat can be removed from the storage by circulating the slurry through a heat exchanger which is part of the cold distribution system. The model of the fluidized bed heat exchanger is coupled with the model of the cold storage system. The phase change process is modelled by using a crystal growth model. The presented work provides experimental data for the thermal performance and hydrate generation of a fluidized bed based LHTS system for a CO2 hydrate cold storage system.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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