Numerical simulation of a solar-assisted ejector air conditioning system with cold storage

Indoor space cooling represents a large potential for solar energy use due to the relative coincidence between energy availability and cooling demand. Solar-assisted air conditioning (AC) applications emerged with the development of high efficiency solar collectors. Energy storage (hot or cold) must be implemented for solar-assisted AC applications when cooling demand is present during intervals without available solar energy and also for cooling capacity optimisation (“peak shaving”).The present paper analyses a solar-assisted ejector cooling system with cold storage. Simulations were carried out over one year considering climatic data for a hot location (Béchar, Algeria) and the performance of the system was assessed for a set of design conditions. Effects of cold storage upon comfort conditions and energy demand were evaluated. Maximum room temperature and overall interval of time during which the room temperature exceeded the set-point value were the parameters used to quantify system performance. It was found that cold storage improved comfort conditions compared to a system without storage. For some design conditions it was found that increasing the cold storage capacity did not result in improved comfort conditions. The control algorithm of the system was identified as the cause of this behaviour.     

Analysis of cool storage for air conditioning

The paper investigates the thermal performance of ice-water cool storage systems theoretically and experimentally. Cool storage systems are used to shift electric demand from periods of high demand to those of low demand. A general lump model to determine the thermal storage characteristics is developed. The solution of the system equations is found through a Laplace transformation method. Experimental data of temperature profiles obtained for both single container and packet containers under the conditions of various flow rates and different inlet coolant temperatures confirm the validity of the theory. A closed form solution of the required time for latent-heat storage has also been established on the basis of the quasi-steady-state assumption of the coolant.     

Performance enhancement in latent heat thermal storage system: A review

Phase change material (PCM) based latent heat thermal storage (LHTS) systems offer a challenging option to be employed as an effective energy storage and retrieval device. The performance of LHTS systems is limited by the poor thermal conductivity of PCMs employed. Successful large-scale utilization of LHTS systems thus depends on the extent to which the performance can be improved. A great deal of work both experimental and theoretical on different performance enhancement techniques has been reported in the literature. This paper reviews the implementation of those techniques in different configurations of LHTS systems. The influence of enhancement techniques on the thermal response of the PCM in terms of phase change rate and amount of latent heat stored/retrieved has been addressed as a main aspect. Issues related to mathematical modeling of LHTS systems employing enhancement techniques are also discussed.     

Performance evaluation of a solar thermal energy storage system using nanoparticle-enhanced phase change material

This paper presents a numerical investigation on the thermal performance of a solar latent heat storage unit composed of rectangular slabs combined with a flat-plate solar collector. The rectangular slabs of the storage unit are vertically arranged and filled with phase change material (PCM: RT50) dispersed with high conductive nanoparticles (Al₂O₃). A heat transfer fluid (HTF: water) goes flow in the solar collector and receives solar thermal energy form the absorber area, then circulates between the slabs to transfer heat by forced convection to nanoparticle-enhanced phase change material (NEPCM). A numerical model based on the finite volume method and the conservation equations was developed to model the heat transfer and flow processes in the storage unit. The developed model was validated by comparing the obtained results with the experimental, numerical and theoretical results published in the literature. The thermal performance of the investigated latent heat storage unit combined with the solar collector was evaluated under the meteorological data of a representative day of the month of July in Marrakesh city, Morocco. The effect of the dispersion of high conductive nanoparticles on the thermal behavior and storage performance was also evaluated and compared with the case of base PCM without additives.     

冰蓄冷空调系统经济性的影响因素

介绍了冰蓄冷空调系统的策略，通过建立冰蓄冷空调系统的经济性模型，进行了经济性优化计算，分析了蓄冷率、峰谷电价比、电力增容费和蓄冷方式等因素对冰蓄冷空调系统在初投资、运行电费和总费用方面的影响。     

Thermal storage in an air conditioning system with dual energy storage unit

在自行搭建的双蓄能实验平台上进行了制冷兼蓄热实验研究,对比了制冷兼蓄热模式和一般制冷模式,探讨了不同冷冻水流量和不同风机盘管风量对机组性能的影响.实验结果表明:蓄热对机组制冷端的影响很小,但是由于回收了大量的冷凝热,使得机组的综合能效比得到大幅提高,因此蓄热对空调节能具有较大作用.此外,在制冷兼蓄热模式下,冷冻水流量或风机盘管风量越大,机组的综合能效比越大,当风量为1033 m³/h,冷冻水流量为972 L/h时,机组综合能效比高达7.06.     

Simulation of heat transfer in the cool storage unit of a liquid–air energy storage system

An energy storage system that stores energy in the form of liquid air was studied. In this system, the cool storage unit was the most important unit. From the viewpoint of safety and economy, it was most promising to store the cold energy as the sensitive heat of a solid such as pebbles or concrete. A simulator was developed to predict temperature variations of the solid cool storage unit. The simulator calculated unsteady heat transfer between a supercritical gas flow and the solid material. Comparison of calculated and experimental results showed that the temperature variation of the metal cool storage medium was accurate within 11%. The calculated results showed for the concrete cool storage unit that a smaller quantity of medium was required with a smaller pitch of the tube. The minimum quantity of concrete calculated at the smallest pitch was three times that of concrete, which was simply estimated from the heat capacity of concrete and air. The volume required for concrete cool storage was less than 1/100 that of reservoirs for a pumped‐hydro power station having a vertical drop of 500 m. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 284–296, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10035     

A hybrid energy storage system using pump compressed air and micro-hydro turbine

In this paper, a micro-hybrid energy storage system, for a small power grid, which combines the concepts of pump storage plant (PSP) and compressed air energy storage (CAES), is proposed. There are two tanks, one open to the air and one subjected to compressed air, as well as a micro-pump turbine (MPT) in the hybrid system. The basic principle is that the MPT utilizes excess power from the grid to pump the water, which in turn compresses the air, and in this way, the energy is changed into internal energy of air. The energy in the air will be released to drive water passing through the MPT to generate power when the supply of power from the grid is insufficient. To validate the above proposal, such a micro-system was designed considering geometrical and operational conditions. Due to the large head variation for MPT, a variable speed machine [1] was designed by means of an inverse design method. After geometrical modeling and mesh generation for the complete configuration of the MPT, which consists of spiral casing, tandem, runner and draft tube, CFD simulations of typical operating points during pump mode and turbine mode were implemented. Special treatments of boundary conditions induced by the air compression or decompression were applied in the simulation. This energy storage system shows promising potential for application as the results indicated that the performance of the system and MPT was comparable.     

Performance prediction of a coupled metal hydride based thermal energy storage system

The present study discusses the thermodynamic compatibility criteria for the selection of metal hydride pairs for the application in coupled metal hydride based thermal energy storage systems. These are closed systems comprising of two metal hydride beds – a primary bed for energy storage and a secondary bed for hydrogen storage. The performance of a coupled system is analyzed considering Mg₂Ni material for energy storage and LaNi₅ material for hydrogen storage. A 3-D model is developed and simulated using COMSOL Multiphysics® at charging and discharging temperatures of 300 °C and 230 °C, respectively. The LaNi₅ bed used for hydrogen storage is operated close to ambient temperature of 25 °C. The results of the first three consecutive cycles are presented. The thermal storage system achieved a volumetric energy storage density of 156 kWh m⁻³ at energy storage efficiency of 89.4% during third cycle.     

某地下商场冰蓄冷低温送风系统的模拟与分析

针对地下商场这样的特殊场所,采用冰蓄冷低温送风的具体情况需要进行特殊的研究与探讨。以成都天府广场地下步行街为研究对象,探讨了冰蓄冷低温送风系统送风温度的优化选择问题。根据地下商业建筑负荷特点,主要考虑人员、设备和照明散热引起的传热过程,建立了采用方形散流器变风量系统结合低温送风的空调房间计算模型,对气流组织进行了数值模拟。     

Performance enhancement of an integrated collector storage hot water system

Integrated collector storage (ICS) systems offer a solution to reduce the height of the conventional flat-plate thermosiphon type collectors. The initial system developed had an aperture area of 1.77 m², a receiver diameter of 200 mm, a concentration ratio of 1.47 and total water storage volume of 65 litres. The main disadvantage of the ICS systems comes from their design, i.e., because the collector absorber is also the storage cylinder it is not possible to insulate it properly and therefore there are significant losses during the night. The main cause of these losses is the convection currents created during the night, circulating around the top glass cover. Another disadvantage of the system is its draw-off characteristics. Because the water cylinder/absorber is horizontal there is very little stratification of the water in the cylinder. It is suggested that a primary 110 mm diameter cylinder is introduced at the space between the main cylinder and the glass. The cold water is introduced directly to the primary cylinder, which feeds the main cylinder. With this modification the convection currents are drastically reduced due to the obstruction created by the primary vessel, thus reducing the night thermal losses. Also as the cold water is introduced first to the primary cylinder there is no direct mixing of the two streams thus greatly improving the system draw-off characteristics. This modification creates an 8% increase in the total cost of the system, which is reasonable, if the above benefits are considered     

Thermal characteristics of packed bed storage system

The thermal behaviour of a packed bed storage system charged with hot air is modelled using two partial differential equations representing the energy conservation in the air and solid phases constituting the bed. These two equations are coupled through the heat exchange process between the two phases. A fully implicit numerical scheme based on forward, upwind and central differencing for the time, first and second space derivatives, respectively, is used to solve the modelling equations. Marching technique is used for the air equation and a tri-diagonal matrix solver is employed to solve the solid equation. The solution yields the thermal structure of the bed, namely the air and solid temperature distribution inside the bed at any particular time, and the variation of total energy stored in the bed with time. The effect of bed length, solid diameter and void fraction on the thermal characteristics of the packed bed is studied. Further, the performance of the bed under variable inlet air temperature and mass flow rate is investigated.     

Heat pipe based cold energy storage systems for datacenter energy conservation

In the present paper, design and economics of the novel type of thermal control system for datacenter using heat pipe based cold energy storage has been proposed and discussed. Two types of cold energy storage system namely: ice storage system and cold water storage system are explained and sized for datacenter with heat output capacity of 8800 kW. Basically, the cold energy storage will help to reduce the chiller running time that will save electricity related cost and decrease greenhouse gas emissions resulting from the electricity generation from non-renewable sources. The proposed cold energy storage system can be retrofit or connected in the existing datacenter facilities without major design changes. Out of the two proposed systems, ice based cold energy storage system is mainly recommended for datacenters which are located in very cold locations and therefore can offer long term seasonal storage of cold energy within reasonable cost. One of the potential application domains for ice based cold energy storage system using heat pipes is the emergency backup system for datacenter. Water based cold energy storage system provides more compact size with short term storage (hours to days) and is potential for datacenters located in areas with yearly average temperature below the permissible cooling water temperature (∼25 °C). The aforesaid cold energy storage systems were sized on the basis of metrological conditions in Poughkeepsie, New York. As an outcome of the thermal and cost analysis, water based cold energy storage system with cooling capability to handle 60% of datacenter yearly heat load will provide an optimum system size with minimum payback period of 3.5 years. Water based cold energy storage system using heat pipes can be essentially used as precooler for chiller. Preliminary results obtained from the experimental system to test the capability of heat pipe based cold energy storage system have provided satisfactory outcomes and validated the proposed system concept.     

蓄热式热泵在空调系统中的应用

讨论了蓄热式热泵的工作原理、特点及设计方法 ,同时介绍了一个实际工程。指出使用蓄热式热泵既可充分合理利用能源 ,降低用户运行费用 ,又为电网提供了一种调荷避峰的好方式。     

Performance study of a partitioned thermally stratified storage tank in a solar powered absorption air conditioning system

Accurate modeling of solar heating or cooling with storage generally requires an accounting of the stratification within such storage tank, since overall system performance is significantly affected by the storage temperature distribution. In this study, a simple one-dimensional multi-node approach, taking into account of the axial heat conduction between nodes, has been used to theoretically analyze temperature stratification in the thermal storage tank. The results indicate that, for less collector area, the heat removal factor plays a major role in increasing the system performance, than the thermal stratification. Also, an optimum ratio of tank volume over collector area exists for a solar powered absorption air conditioning system. This paper also reviews the state of the art on different kinds of variable inlet design, and a simple new inlet design (partitioning the tank) has been introduced to effect better thermal stratification in storage tank.     

Simulating reservoir storage for a wind-hydro hydrid system

A wind-hydro hybrid system is proposed to improve Brazil's use of its renewable energy portfolio. In this scheme, winds from local or remote locations supply extra generation, so that the hydroelectric outflow can be reduced accordingly to wind production. The water savings, in turn, lead to a significant improvement of the reservoir's storage. A simple model is developed and applied to the Itumbiara hydroelectric reservoir of midwest Brazil. Scenarios for different wind contributions are compared to historical observations from 1994 to 2011. Results suggest that, if implemented, the hybrid system should improve the country's energy security. Hydroelectric reservoirs might be able to confront interannual climate variability without risks to the energy supply. A positive impact on the multiple use of reservoirs is expected for fish-farming, irrigation, recreation, navigation and water supply.     

Performance studies of a double glazed augmented integrated rock system

This paper describes the design details of and experimental investigations on a prototype augmented integrated rock system (AIRS) with an intention of using solar energy for crop drying and space heating. The system investigated is a flat plate solar collector of conventional design connected in series with rock storage and a collection unit. For the evaluation of the 24 hour performance of this system, outdoor calorimetric tests have been performed. The experimental results for varying inlet air flow rate have been analysed and the final results have been included. AIRS' overall daily efficiency is determined by testing the system outdoors and monitoring the output energy and the incident solar radiation. Taking into consideration the total cost of the system it has been designed so that it can be fabricated on site using, wherever possible, readily available tools and materials.     

Experimental study and analysis of the application of ice‐storage capsules in an air conditioning system

Ice‐storage capsules are used in an energy‐efficient air conditioning system. Experiments and numerical simulations were conducted to investigate the effects of different structural configurations of the capsules on the freezing and melting processes of the refrigerant solution sealed inside the capsules. The predictions of the numerical simulation are in good agreement with the test results. The results show that the freezing speed of the capsules equipped with a metal core is 30% to 50% faster than for capsules without the metal core. In addition, it was found that the special refrigerant solution used in the experiment had a lower freezing point and a higher freezing speed than water. The effects of the size of the metal core, and the coolant temperatures are also investigated. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 21–27, 2002     

Separation characteristics of clathrate hydrates from a cooling plate for efficient cold energy storage

To improve the coefficient of performance (COP) in air-conditioning systems, the liquid–solid phase change temperature of the cold energy storage material should be approximately 10 °C. Moreover, a thermal storage material that forms a slurry can maintain large heat capacity for the working fluids. Solids that adhere to the heat transfer surface form a thermal resistance layer and significantly reduce the cooling storage rate; therefore, it is important to avoid adhesion of a thick solid layer on the surface to realize efficient energy storage.