Modelling and performances of a deep-freezing process using low-grade solar heat

A solar deep-freezing process has been designed. It aims at cooling down a cold box to about −20 °C, using simple flat plate solar collectors operating at 70 °C. This original process involves two cascaded thermochemical systems based on the BaCl₂/ammonia reaction. Its working mode is discontinuous as it alternates between a regeneration mode during daytime and a cold production mode during nighttime. A global dynamic model involving the various system components allows the simulation of the process; it predicts the evolution of the components temperatures and the rates of chemical reactions of the system. It also allows the dimensioning of the system components to maintain a 500 l cold box at −20 °C during the 6 sunniest months of the year under typical Mediterranean weather conditions and provide over 80% of the total yearly cooling needs of this box. This requires a solar collector area of 5.8 m² and 39 kg of reactive salt. The predicted coefficient of performance (COP) is about 0.1 over the year, and the net solar COP, taking into account the collector efficiencies, is 0.05.     

Modelling and performance study of a continuous adsorption refrigeration system driven by parabolic trough solar collector

This article suggests a numerical study of a continuous adsorption refrigeration system consisting of two adsorbent beds and powered by parabolic trough solar collector (PTC). Activated carbon as adsorbent and ammonia as refrigerant are selected. A predictive model accounting for heat balance in the solar collector components and instantaneous heat and mass transfer in adsorbent bed is presented. The validity of the theoretical model has been tested by comparison with experimental data of the temperature evolution within the adsorber during isosteric heating phase. A good agreement is obtained. The system performance is assessed in terms of specific cooling power (SCP), refrigeration cycle COP (COP_cycle) and solar coefficient of performance (COP_s), which were evaluated by a cycle simulation computer program. The temperature, pressure and adsorbed mass profiles in the two adsorbers have been shown. The influences of some important operating and design parameters on the system performance have been analyzed.The study has put in evidence the ability of such a system to achieve a promising performance and to overcome the intermittence of the adsorption refrigeration systems driven by solar energy. Under the climatic conditions of daily solar radiation being about 14 MJ per 0.8 m² (17.5 MJ/m²) and operating conditions of evaporating temperature, T_ev = 0 °C, condensing temperature, T_con = 30 °C and heat source temperature of 100 °C, the results indicate that the system could achieve a SCP of the order of 104 W/kg, a refrigeration cycle COP of 0.43, and it could produce a daily useful cooling of 2515 kJ per 0.8 m² of collector area, while its gross solar COP could reach 0.18.     

Experimental investigation of a natural zeolite–water adsorption cooling unit

In this study, a thermally driven adsorption cooling unit using natural zeolite–water as the adsorbent–refrigerant pair has been built and its performance investigated experimentally at various evaporator temperatures. The primary components of the cooling unit are a shell and tube adsorbent bed, an evaporator, a condenser, heating and cooling baths, measurement instruments and supplementary system components. The adsorbent bed is considered to enhance the bed’s heat and mass transfer characteristics; the bed consists of an inner vacuum tube filled with zeolite (zeolite tube) inserted into a larger tubular shell. Under the experimental conditions of 45 °C adsorption, 150 °C desorption, 30 °C condenser and 22.5 °C, 15 °C and 10 °C evaporator temperatures, the COP of the adsorption cooling unit is approximately 0.25 and the maximum average volumetric cooling power density (SCP_v) and mass specific cooling power density per kg adsorbent (SCP) of the cooling unit are 5.2 kW/m³ and 7 W/kg, respectively.     

An investigation of the solar powered absorption refrigeration system with advanced energy storage technology

This paper presented a new solar powered absorption refrigeration (SPAR) system with advanced energy storage technology. The advanced energy storage technology referred to the Variable Mass Energy Transformation and Storage (VMETS) technology. The VMETS technology helped to balance the inconsistency between the solar radiation and the air conditioning (AC) load. The aqueous lithium bromide (H₂O-LiBr) was used as the working fluid in the system. The energy collected from the solar radiation was first transformed into the chemical potential of the working fluid and stored in the system. Then the chemical potential was transformed into thermal energy by absorption refrigeration when AC was demanded. In the paper, the working principle and the flow of the SPAR system were explained and the dynamic models for numerical simulation were developed. The numerical simulation results can be used to investigate the behavior of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the heat loads of heat exchanger devices and so on. An example was given in the paper. In the example, the system was used in a subtropical city like Shanghai in China and its operating conditions were set as a typical summer day: the outdoor temperature varied between 29.5 °C and 38 °C, the maximum AC load was 15.1 kW and the total AC capacity was 166.1 kW h (598.0 MJ). The simulation results indicated that the coefficient of performance (COP) of the system was 0.7525 or 0.7555 when the condenser was cooled by cooling air or by cooling water respectively and the storage density (SD) was about 368.5 MJ/m³. As a result, the required solar collection area was 66 m² (cooling air) or 62 m² (cooling water) respectively. The study paves the road for system design and operation control in the future.     

Experimental performance analysis on a direct-expansion solar-assisted heat pump water heater

A direct expansion solar assisted heat pump water heater (DX-SAHPWH) experimental set-up is introduced and analyzed. This DX-SAHPWH system mainly consists of 4.20 m² direct expansion type collector/evaporator, R-22 rotary-type hermetic compressor with rated input power 0.75 kW, 150 L water tank with immersed 60 m serpentine copper coil and external balance type thermostatic expansion valve. The experimental research under typical spring climate in Shanghai showed that the COP of the DX-SAHPWH system can reach 6.61 when the average temperature of 150 L water is heated from 13.4 °C to 50.5 °C in 94 min with average ambient temperature 20.6 °C and average solar radiation intensity 955 W/m². And the COP of the DX-SAHPWH system is 3.11 even if at a rainy night with average ambient temperature 17.1 °C. The seasonal average value of the COP and the collector efficiency was measured as 5.25 and 1.08, respectively. Through exergy analysis for each component of the DX-SAHPWH system, it can be calculated that the highest exergy loss occurs in the compressor, followed by collector/evaporator, condenser and expansion valve, respectively. Further more, some methods are suggested to improve the thermal performance of each component and the whole DX-SAHPWH system.     

Cooling cogeneration cycle

The existed combined power and cooling cycle operates with ammonia–water mixture as working fluid having low cooling due to the vapor at the inlet of evaporator. It also demands high ammonia concentration at turbine inlet to get cooling and suitable only at low sink temperature (10–12°C). A new cooling cogeneration cycle has been proposed and solved to generate more cooling with adequate power generation from single source of heat with two options in working fluids i.e. ammonia–water mixture and LiBr–water mixture. The results show that an increase in cycle maximum temperature is only supporting the power but not the cooling. A suitable range for separator temperature has been developed and optimized to maximize the total output. From this study, the resulted specific power, specific cooling, cycle power efficiency, cycle coefficient of performance (COP) and cycle energy utilization factor (EUF), plant EUF, and specific area of solar collector are 0.008 kW/m², 0.11 kW/m², 2%, 0.28, 0.3, 0.13 and 8 m²/kW for ammonia–water cycle and 0.04 kW/m², 0.3 kW/m², 9.5%, 0.7, 0.8, 0.37 and 3 m²/kW for LiBr–water mixture plant respectively.     

Performance assessment of an integrated free cooling and solar powered single-effect lithium bromide-water absorption chiller

In this study, performance assessment of an integrated cooling plant having both free cooling system and solar powered single-effect lithium bromide–water absorption chiller in operation since August 2002 in Oberhausen, Germany, was performed. A floor space of 270 m² is air-conditioned by the plant. The plant includes 35.17 kW cooling (10-RT) absorption chiller, vacuum tube collectors’ aperture area of 108 m², hot water storage capacity of 6.8 m³, cold water storage capacity of 1.5 m³ and a 134 kW cooling tower. The results show that free cooling in some cooling months can be up to 70% while it is about 25% during the 5 years period of the plant operation. For sunny clear sky days with equal incident solar radiation, the daily solar heat fraction ranged from 0.33 to 0.41, collectors’ field efficiency ranged from 0.352 to 0.492 and chiller COP varies from 0.37 to 0.81, respectively. The monthly average value of solar heat fraction varies from 31.1% up to 100% and the five years average value of about 60%. The monthly average collectors’ field efficiency value varies from 34.1% up 41.8% and the five-year average value amounts about 28.3%. Based on the obtained results, the specific collector area is 4.23 (m²/kW_cold) and the solar energy system support of the institute heating system for the duration from August 2002 to November 2007 is 8124 kWh.     

Solar absorption cooling plant in Seville

A solar/gas cooling plant at the Engineering School of Seville (Spain) was tested during the period 2008-2009. The system is composed of a double-effect LiBr + water absorption chiller of 174 kW nominal cooling capacity, powered by: (1) a pressurized hot water flow delivered by mean of a 352 m² solar field of a linear concentrating Fresnel collector and (2) a direct-fired natural gas burner. The objective of the project is to indentify design improvements for future plants and to serve as a guideline. We focused our attention on the solar collector size and dirtiness, climatology, piping heat losses, operation control and coupling between solar collector and chiller. The daily average Fresnel collector efficiency was 0.35 with a maximum of 0.4. The absorption chiller operated with a daily average coefficient of performance of 1.1-1.25, where the solar energy represented the 75% of generator’s total heat input, and the solar cooling ratio (quotient between useful cooling and insolation incident on the solar field) was 0.44.     

Intergration of evacuated tubular solar collectors with lithium bromide absorption cooling systems

By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 per cent under conditions of ΔT/H_T = 75°C m²/kW (ΔT = collector fluid inlet temperature minus ambient temperature, H_T = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70–88°C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors intergrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III.     

System performance and economic analysis of solar-assisted cooling/heating system

The long-term system simulation and economic analysis of solar-assisted cooling/heating system (SACH-2) was carried out in order to find an economical design. The solar heat driven ejector cooling system (ECS) is used to provide part of the cooling load to reduce the energy consumption of the air conditioner installed as the base-load cooler. A standard SACH-2 system for cooling load 3.5 kW (1 RT) and daily cooling time 10 h is used for case study. The cooling performance is assumed only in summer seasons from May to October. In winter season from November to April, only heat is supplied. Two installation locations (Taipei and Tainan) were examined.It was found from the cooling performance simulation that in order to save 50% energy of the air conditioner, the required solar collector area is 40 m² in Taipei and 31 m² in Tainan, for COP_j = 0.2. If the solar collector area is designed as 20 m², the solar ejector cooling system will supply about 17–26% cooling load in Taipei in summer season and about 21–27% cooling load in Tainan. Simulation for long-term performance including cooling in summer (May–October) and hot water supply in winter (November–April) was carried out to determine the monthly-average energy savings. The corresponding daily hot water supply (with 40 °C temperature rise of water) for 20 m² solar collector area is 616–858 L/day in Tainan and 304–533 L/day in Taipei.The economic analysis shows that the payback time of SACH-2 decreases with increasing cooling capacity. The payback time is 4.8 years in Tainan and 6.2 years in Taipei when the cooling capacity >10 RT. If the ECS is treated as an additional device used as a protective equipment to avoid overheating of solar collectors and to convert the excess solar heat in summer into cooling to reduce the energy consumption of air conditioner, the payback time is less than 3 years for cooling capacity larger than 3 RT.     

Experience with fully operational solar-driven 10-ton LiBr/H2O single-effect absorption cooling system in Thailand

A solar-driven 10-ton LiBr/H₂O single-effect absorption cooling system has been designed and installed at the School of Renewable Energy Technology (SERT), Phitsanulok, Thailand. Construction took place in 2005, after which this system became fully operational and has been supplying cooling for our main testing building's air-conditioning. Data on the system's operation were collected during 2006 and analyzed to find the extent to which solar energy replaced conventional energy sources. Here, we present these data and show that the 72 m² evacuated tube solar collector delivered a yearly average solar fraction of 81%, while the remaining 19% of thermal energy required by the chiller was supplied by a LPG-fired backup heating unit. We also show that the economics of this cooling system are dominated by the initial cost of the solar collector array and the absorption chiller, which are significantly higher than that of a similar-size conventional VCC system.     

Development of a new flat stationary evacuated CPC-collector for process heat applications

For the economical supply of solar process heat at temperatures between 120 and 150 °C a new non-tracking, flat, low-concentrating collector has been developed. The new collector is an edge ray collector with a concentration of 1.8 and inert gas filling, existing of parallel mounted absorber-reflector units, aligned in east-west direction. The basic concept is the integration of an absorber tube and reflectors inside a low pressure enclosure. Asymmetrical reflectors below the headers with a concentration of 0.6X provide extra radiation and prevent longitudinal radiation losses. To suppress heat losses due to gas-convection inside, air or inert gas like krypton at a pressure below 10 mbar is used.A prototype, with an aperture area of 2.0 m², was tested in Munich and showed efficiencies of about 50% for krypton at 0.01 bar at a temperature of 150 °C with a radiation of 1000 W/m² (900 W/m² direct, ambient temperature 20 °C).     

Simulation and optimization of a LiBr solar absorption cooling system with evacuated tube collectors

Solar radiation is a clean form of energy, which is required for almost all natural processes on earth. Solar-powered air-conditioning has many advantages when compared to a conventional electrical system. This paper presents a solar cooling system that has been designed for Malaysia and similar tropical regions using evacuated tube solar collectors and LiBr absorption unit. The modeling and simulation of the absorption solar cooling system is carried out with TRNSYS program. The typical meteorological year file containing the weather parameters for Malaysia is used to simulate the system. The results presented show that the system is in phase with the weather, i.e. the cooling demand is large during periods that the solar radiation is high. In order to achieve continuous operation and increase the reliability of the system, a 0.8 m³ hot water storage tank is essential. The optimum system for Malaysia's climate for a 3.5 kW (1 refrigeration ton) system consists of 35 m² evacuated tubes solar collector sloped at 20°.     

Adsorption cooling driven by solar collector: A case study for Tokyo solar data

An analytical investigation has been performed to study the possibility of application of solar cooling for the climatic condition of Tokyo, Japan. Silica gel–water adsorption cooling system has been taken into consideration for the present study and lumped parameter model is used to investigate the performance of the system. Based on the solar radiation data it is found that at least 15 collector (each of 2.415 m²) is required to achieve the required heat source temperature (around 85 °C) to run the cooling unit. It is also observed that the solar powered adsorption cooling unit provides cooling capacity around 10 kW at noon with base run conditions, while the system provides solar COP around 0.3, however, the solar collector size can be reduced by optimizing the cycle time.     

Convective boiling heat transfer characteristics of CO2 in microchannels

Convective boiling heat transfer coefficients and dryout phenomena of CO₂ are investigated in rectangular microchannels whose hydraulic diameters range from 1.08 to 1.54 mm. The tests are conducted by varying the mass flux of CO₂ from 200 to 400 kg/m² s, heat flux from 10 to 20 kW/m², while maintaining saturation temperature at 0, 5 and 10 °C. Test results show that the average heat transfer coefficient of CO₂ is 53% higher than that of R134a. The effects of heat flux on the heat transfer coefficient are much significant than those of mass flux. As the mass flux increases, dryout becomes more pronounced. As the hydraulic diameter decreases from 1.54 to 1.27 mm and from 1.27 to 1.08 mm at a heat flux of 15 kW/m² and a mass flux of 300 kg/m² s, the heat transfer coefficients increase by 5% and 31%, respectively. Based on the comparison of the data from the existing models with the present data, the Cooper model and the Gorenflo model yield relatively good predictions of the measured data with mean deviations between predicted and measured data of 21.7% and 21.2%, respectively.     

A solar thermal cooling and heating system for a building: Experimental and model based performance analysis and design

A solar thermal cooling and heating system at Carnegie Mellon University was studied through its design, installation, modeling, and evaluation to deal with the question of how solar energy might most effectively be used in supplying energy for the operation of a building. This solar cooling and heating system incorporates 52 m² of linear parabolic trough solar collectors; a 16 kW double effect, water-lithium bromide (LiBr) absorption chiller, and a heat recovery heat exchanger with their circulation pumps and control valves. It generates chilled and heated water, dependent on the season, for space cooling and heating. This system is the smallest high temperature solar cooling system in the world. Till now, only this system of the kind has been successfully operated for more than one year. Performance of the system has been tested and the measured data were used to verify system performance models developed in the TRaNsient SYstem Simulation program (TRNSYS). On the basis of the installed solar system, base case performance models were programmed; and then they were modified and extended to investigate measures for improving system performance. The measures included changes in the area and orientation of the solar collectors, the inclusion of thermal storage in the system, changes in the pipe diameter and length, and various system operational control strategies. It was found that this solar thermal system could potentially supply 39% of cooling and 20% of heating energy for this building space in Pittsburgh, PA, if it included a properly sized storage tank and short, low diameter connecting pipes. Guidelines for the design and operation of an efficient and effective solar cooling and heating system for a given building space have been provided.     

Adsorption refrigeration—An efficient way to make good use of waste heat and solar energy

This paper presents the achievements gained in solid sorption refrigeration prototypes since the end of the l970s, when interest in sorption systems was renewed. The applications included are ice making and air conditioning. The latter includes not only cooling and heating, but also dehumidification by desiccant systems. The prototypes presented were designed to use waste heat or solar energy as the main heat source. The waste heat could be from diesel engines or from power plants, in combined cooling, heating and power systems (CCHP). The current technology of adsorption solar-powered icemakers allows a daily ice production of between 4 and 7 kg m⁻² of solar collector, with a solar coefficient of performance (COP) between 0.10 and 0.16. The silica gel–water chillers studied can be powered by hot water warmer than 55 °C. The COP is usually around 0.2–0.6, and in some commercially produced machines, it can be up to 0.7. The utilization of such chillers in CCHP systems, hospitals, buildings and grain depots are discussed. Despite their advantages, solid sorption systems still present some drawbacks such as low specific cooling power (SCP) and COP. Thus, some techniques to overcome these problems are also contemplated, together with the perspectives for their broad commercialisation. Among these techniques, a special attention was devoted to innovative adsorbent materials, to advanced cycles and to heat pipes, which are suitable devices not only to improve the heat transfer but also can help to avoid corrosion in the adsorbers. Recent experiments performed by the research group of the authors with machines that employ composite adsorbent material and heat pipes showed that it is possible to achieve a SCP of 770 W kg⁻¹ of salt and COP of 0.39 at evaporation temperatures of −20 °C and generation temperature of 115 °C.     

New deep-freezing process using renewable low-grade heat: From the conceptual design to experimental results

An exergy-based analysis applied to ideal thermochemical dipoles allowed to design an original process that could use low-grade energy, produced from a thermal solar collector at around 70 °C, to provide low-temperature cold, below −23 °C, in order to store deep-frozen food. The ideal coefficient of performance (COP) of this system is 0.5 and the exergetic yield is 1. Taking into account the process enthalpies and the sensible heat of the reactants, the COP_thermo is 0.17. The process functioning is described in this paper. It alternates between a regeneration mode during daytime and cold production mode during night-time. An experimental prototype was designed and built. It proved the feasibility of the concept and showed an experimental COP of about 0.06, which is similar to the up-to-date solar cooling systems, but at higher cold temperatures. The mean annual exergetic yield of the process is about 0.06.     

Water immersion cooling of PV cells in a high concentration system

Temperature control of solar cells at high concentrations is a key issue. Short-term efficiency drop and long-term degradation should be avoided by effective cooling methods. Liquid immersion cooling eliminates the contact thermal resistance of back cooling and should improve cell performance. A 250X dish concentrator with two-axis tracking was utilized to evaluate a new CPV system using de-ionized water for immersion cooling. Time-dependent temperature distributions of the PV module of high power back point-contact cells were measured, as well as the I-V curves. The cooling capacities of the liquid immersion approach are very favorable. The module temperature can be cooled to 45 °C at a 940 W/m² direct normal irradiance, 17 °C ambient temperature and 30 °C water inlet temperature. The temperature distribution of the module is quite uniform, but the electrical performance of the cell module degrades after a fairly long time immersion in the de-ionized water.     

Dye-sensitized, nano-porous TiO2 solar cell with poly(acrylonitrile): MgI2 plasticized electrolyte

Dye-sensitized solar cells are promising candidates as supplementary power sources; the dominance in the photovoltaic field of inorganic solid-state junction devices is in fact now being challenged by the third generation of solar cells based on dye-sensitized, nano-porous photo-electrodes and polymer electrolytes. Polymer electrolytes are actually very favorable for photo-electrochemical solar cells and in this study poly(acrylonitrile)-MgI₂ based complexes are used. As ambient temperature conductivity of poly(acrylonitrile)-salt complexes are in general low, a conductivity enhancement is attained by blending with the plasticizers ethylene carbonate and propylene carbonate. At 20 °C the optimum ionic conductivity of 1.9 × 10⁻³ S cm⁻¹ is obtained for the (PAN)₁₀(MgI₂)_n(I₂)_n/10(EC)₂₀(PC)₂₀ electrolyte where n = 1.5. The predominantly ionic nature of the electrolyte is seen from the DC polarization data. Differential scanning calorimetric thermograms of electrolyte samples with different MgI₂ concentrations were studied and glass transition temperatures were determined. Further, in this study, a dye-sensitized solar cell structure was fabricated with the configuration Glass/FTO/TiO₂/Dye/Electrolyte/Pt/FTO/Glass and an overall energy conversion efficiency of 2.5% was achieved under solar irradiation of 600 W m⁻². The I-V characteristics curves revealed that the short-circuit current, open-circuit voltage and fill factor of the cell are 3.87 mA, 659 mV and 59.0%, respectively.