Waste heat driven silica gel/water adsorption cooling in trigeneration

A low-grade waste heat driven solid/vapour adsorption chiller has been successfully designed and tested. A simple model was developed to aid the design and predict the performances. The system comprised two identical sorption reactors operating out of phase in order to ensure continuous cold production. One sorption reactor consisted of six commercially available automotive plate/fin heat exchangers in which silica gel grains were accommodated between the fins. The system was tested as to the power delivered at 12 °C and the power density. The average cooling power was 3.6 kW. This is only 72% of the design value and can be largely attributed to the lower heat transfer fluid flow rate through the sorbent reactor. The thermal efficiency, COP, was 0.62 and the power density was 17 kW/m³ for the system as a whole. Higher power densities are possible. At present, the adsorption chiller is integrated in a prototype trigeneration system, which is tested at CRF’s Eco-building in Turin.     

Experimentation on a cogenerative system based on a microturbine

The paper reports on experimental results of an energetic characterization of a cogenerative plant. The generator is a microturbine Turbec T100-CHP integrated in a heat recovery system. For operation in standard conditions the maximum electrical and thermal power generated are, respectively, 105 and 167 kW. Experimental tests were run by varying the electrical power produced between 50 and 110 kW with 10 kW stepping. For each step the set-point of the water temperature at the outlet of the recuperator was varied in the range 60–80 °C with 5 °C stepping. In every operating condition the measurement system allows the real-time calculation of the quantities needed for the energetic characterization of the plant, such as efficiency indices and PES (primary energy saving index). It is seen that performances remain essentially constant in the range 80–110 kW. A moderate decrease is then observed until about 60 kW, while a further reduction of the electrical power implies a clear worsening (PES decreases from about 30% to 16% in the tested range). Furthermore, environmental impact has been investigated with respect to gaseous and acoustic emissions. In particular the concentration of pollutants in exhaust gases, except for NO_x and CO₂, strongly increases by reducing the electrical power output.     

A new methodology of studying the dynamics of water sorption/desorption under real operating conditions of adsorption heat pumps: Experiment

In this paper we proposed and tested a new methodology of studying the kinetics of water vapour sorption/desorption under operating conditions typical for isobaric stages of sorption heat pumps. The measurements have been carried out on pellets of composite sorbent SWS-1L (CaCl₂ in silica KSK) placed on a metal plate. Temperature of the plate was changed as it takes place in real sorption heat pumps, while the vapour pressure over the sorbent was maintained almost constant (saturation pressures corresponding to the evaporator temperature of 5 °C and 10 °C and the condenser temperature of 30 °C and 35 °C). Near-exponential behaviour of water uptake on time was found for most of the experimental runs. Characteristic time τ of isobaric adsorption (desorption) was measured for one layer of loose grains having a size between 1.4 mm and 1.6 mm for different heating/cooling scenarios and boundary conditions of an adsorption heat pump. Maximum specific power estimated from the τ-values can exceed 1.0 kW/kg of dry adsorbent, that gives proof to the idea of compact adsorption units for energy transformation with loose SWS grains.     

Heat recovery from Diesel engines: A thermodynamic comparison between Kalina and ORC cycles

In the context of heat recovery for electric power generation, Kalina cycle (a thermodynamic cycle using as working fluid a mixture of water and ammonia) and Organic Rankine Cycle (ORC) represent two different eligible technologies. In this work a comparison between the thermodynamic performances of Kalina cycle and an ORC cycle, using hexamethyldisiloxane as working fluid, was conducted for the case of heat recovery from two Diesel engines, each one with an electrical power of 8900 kWe. The maximum net electric power that can be produced exploiting the heat source constituted by the exhaust gases mass flow (35 kg/s for both engines, at 346 °C) was calculated for the two thermodynamic cycles. Owing to the relatively low useful power, for the Kalina cycle a relatively simple plant layout was assumed. Supposing reasonable design parameters and a logarithmic mean temperature difference in the heat recovery exchanger of 50 °C, a net electric power of 1615 kW and of 1603 kW respectively for the Kalina and for the ORC cycle was calculated.Although the obtained useful powers are actually equal in value, the Kalina cycle requires a very high maximum pressure in order to obtain high thermodynamic performances (in our case, 100 bar against about 10 bar for the ORC cycle). So, the adoption of Kalina cycle, at least for low power level and medium–high temperature thermal sources, seems not to be justified because the gain in performance with respect to a properly optimized ORC is very small and must be obtained with a complicated plant scheme, large surface heat exchangers and particular high pressure resistant and no-corrosion materials.     

Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems

Selection of an appropriate HTF is important for minimising the cost of the solar receiver, thermal storage and heat exchangers, and for achieving high receiver and cycle efficiencies. Current molten salt HTFs have high melting points (142–240 °C) and degrade above 600 °C. Sodium’s low melting point (97.7 °C) and high boiling point (873 °C) allow for a much larger range of operational temperatures. Most importantly, the high temperatures of sodium allow the use of advanced cycles (e.g. combined Brayton/Rankine cycles). In this study, a comparison between the thermophysical properties of two heat transfer fluids (HTFs), Hitec (a ternary molten salt 53% KNO₃ + 40% NaNO₂ + 7% NaNO₃) and liquid sodium (Na), has been carried out to determine their suitability for use in high-temperature concentrated solar thermal central-receiver systems for power generation. To do this, a simple receiver model was developed to determine the influences of the fluids’ characteristics on receiver design and efficiency. While liquid sodium shows potential for solar thermal power systems due to its wide range of operation temperatures, it also has two other important differences – a high heat transfer coefficient (～an order of magnitude greater than Hitec) and a low heat capacity (30–50% lower than Hitec salt). These issues are studied in depth in this model. Overall, we found that liquid sodium is potentially a very attractive alternative to molten salts in next generation solar thermal power generation if its limitations can be overcome.     

Experimental analysis of flow and heat transfer in a miniature porous heat sink for high heat flux application

A novel miniature porous heat sink system was presented for dissipating high heat fluxes of electronic device, and its operational principle and characteristics were analyzed. The flow and heat transfer of miniature porous heat sink was experimentally investigated at high heat fluxes. It was observed that the heat load of up to 280 W (heat flux of 140 W/cm²) was removed by the heat sink with the coolant pressure drop of about 34 kPa across the heat sink system and the heater junction temperature of 62.9 °C at the coolant flow rate of 6.2 cm³/s. Nu number of heat sink increased with the increase of Re number, and maximum value of 323 for Nu was achieved at highest Re of 518. The overall heat transfer coefficient of heat sink increased with the increase of coolant flow rate and heat load, and the maximal heat transfer coefficient was 36.8 kW(m² °C)^?1 in the experiment. The minimum value of 0.16 °C/W for the whole thermal resistance of heat sink was achieved at flow rate of 6.2 cm³/s, and increasing coolant flow rate and heat fluxes could lead to the decrease in thermal resistance. The micro heat sink has good performance for electronics cooling at high heat fluxes, and it can improve the reliability and lifetime of electronic device.     

The effect of cycle boundary conditions and adsorbent grain size on the water sorption dynamics in adsorption chillers

The aim of this work was an experimental study of the temporal evolution of isobaric adsorption uptake (release) for simplest configuration of an adsorbent-heat exchanger unit, namely, a monolayer of loose adsorbent grains located on a metal plate. The study was performed by a large temperature jump method at four various boundary conditions of an adsorptive heat transformation cycle typical for air-conditioning application driven by low temperature heat: T_e = 5 and 10 °C, T_c = 30 and 35 °C and T_HS = 80 °C. The size of the Fuji silica grains was varied from 0.2 to 1.8 mm to investigate its effect on water sorption dynamics. For each boundary set and grain size the experimental kinetic curve could be described by an exponential function up to 80–90% of the equilibrium conversion. Desorption runs are found to be faster than appropriate adsorption runs by a factor of 2.2–3.5, hence, for optimal durations of the isobaric ad- and desorption phases of the chilling cycle should be selected accordingly. The size R of the adsorbent grains was found to be a powerful tool to manage the dynamics of isobaric water ad-/desorption. For large grains the characteristic time was strongly dependent on the grain size and proportional to R². Much less important appeared to be an impact of the boundary conditions which variation just weakly affected the dimensionless kinetic curves for the four tested cycles. The maximal specific cooling/heating power was proportional to the maximal temperature difference ΔT and the contact area S between the layer and the metal plate, and can exceed 10 kW/kg. The heat transfer coefficient α estimated from this power was as large as 100–250 W/(m² K) that much exceeds the value commonly used to describe the cycle dynamics.     

Portable proton exchange membrane fuel-cell systems for outdoor applications

A hydrogen fuelled, 30 W proton exchange membrane fuel-cell (PEMFC) system is presented that is able to operate at an ambient temperature between −20 and 40 °C. The system, which comprises the fuel-cell stack, pumps, humidifier, valves and blowers is fully characterized in a climatic chamber under various ambient temperatures. Successful cold start-up and stable operation at −20 °C are reported as well as the system behaviour during long-term at 40 °C. A simple thermal model of the stack is developed and validated, and accounts for heat losses by radiation and convection. Condensation of steam is addressed as well as reaction gas depletion. The stack is regarded as a uniform heat source. The electrochemical reaction is not resolved. General design rules for the cold start-up of a portable fuel-cell stack are deduced by the thermal model and are taken into consideration for the design. The model is used for a comparison between active-assisted cold start-up procedures with a passive cold start-up from temperatures below 0 °C. It is found that a passive cold start-up may not be the most efficient strategy. Additionally, the influence of different stack concepts on the start-up behaviour is analysed by the thermal model. Three power classes of PEMFC stacks are compared: a Ballard Mk902 module for automotive applications with 85 kW, the forerunner stack Ballard Mk5 (5 kW) for medium power applications, and the developed OutdoorFC stack (30 W), for portable applications.     

Sintered porous heat sink for cooling of high-powered microprocessors for server applications

This paper experimentally investigates the sintered porous heat sink for the cooling of the high-powered compact microprocessors for server applications. Heat sink cold plate consisted of rectangular channel with sintered porous copper insert of 40% porosity and 1.44 × 10^?11 m² permeability. Forced convection heat transfer and pressure drop through the porous structure were studied at Re ? 408 with water as the coolant medium. In the study, heat fluxes of up to 2.9 MW/m² were successfully removed at the source with the coolant pressure drop of 34 kPa across the porous sample while maintaining the heater junction temperature below the permissible limit of 100 ± 5 °C for chipsets. The minimum value of 0.48 °C/W for cold plate thermal resistance (R_cp) was achieved at maximum flow rate of 4.2 cm³/s in the experiment. For the designed heat sink, different components of the cold plate thermal resistance (R_cp) from the thermal footprint of source to the coolant were identified and it was found that contact resistance at the interface of source and cold plate makes up 44% of R_cp and proved to be the main component. Convection resistance from heated channel wall with porous insert to coolant accounts for 37% of the R_cp. With forced convection of water at Re = 408 through porous copper media, maximum values of 20 kW/m² K for heat transfer coefficient and 126 for Nusselt number were recorded. The measured effective thermal conductivity of the water saturated porous copper was as high as 32 W/m K that supported the superior heat augmentation characteristics of the copper–water based sintered porous heat sink. The present investigation helps to classify the sintered porous heat sink as a potential thermal management device for high-end microprocessors.     

A new type adsorber for adsorption ice maker on fishing boats

A new type of adsorber for an adsorption ice maker on fishing boats, which uses a compound adsorbent (activated carbon and CaCl₂) and ammonia working pair, is designed. This type of heat pipe adsorber solves the problem of incompatibility between ammonia, copper, seawater and steel. The heating/cooling power for the adsorption/desorption process of the adsorbent, which is required to be transferred by one heat pipe in the adsorber, is computed by the test results of the adsorbent, and the heat transfer performance of one heat pipe in the adsorber is simulated according to the theory of the two phase closed thermosyphon. The heat transfer performance of the heat pipe can meet the heat demands for adsorption/desorption of the adsorbent when the evaporating temperature is −15 °C and the cycle time is 10 min. A test unit is set up to test the heating/cooling performance of the heat pipe type adsorber, and the experimental results are coincident with the simulation. The performance of a two bed adsorption ice maker with heat pipe adsorbers is predicted, and the cooling power is about 17.1–17.8 kW at the evaporating temperature of −15 °C and cycle time of 10 min with mass recovery between two 29 kg compound adsorbent beds.     

Study on a re-heat two-stage adsorption chiller – The influence of thermal capacitance ratio,overall thermal conductance ratio and adsorbent mass on system performance

Silica gel/water based adsorption cycles have a distinct advantage in their ability to be driven by heat of near-ambient temperature so that waste heat below 100 °C can be recovered. One interesting feature of refrigeration cycles driven by waste heat is that they do not use primary energy as driving source. From this context, some researchers investigated the performance of multi-stage adsorption refrigeration cycles those can be operated by heat source of temperature 60 °C or lower which are usually purged to the environment, with a heat sink of temperature at 30 °C. However, the performances of multi-stage systems are low. To improve system performance, an analytic investigation on a re-heat two-stage chiller is performed to clarify the effect of thermal capacitance ratio of the adsorbent and inert material of sorption element, overall thermal conductance ratio of sorption element and evaporator along with silica gel mass on the chiller performance. Results show that cycle performance is strongly influenced by the sorption elements overall thermal conductance values due to their severe sensible heating and cooling requirements resulting from batched cycle operation. The effect of thermal capacitance ratio (C_s/C_m) becomes significant with relatively higher mass of silica gel. It is also found that the chiller performance increases significantly in the range of silica gel mass from 4 to 20 kg.     

Experimental study of a solid adsorption cooling system using flat-tube heat exchangers as adsorption bed

In this paper, a solid adsorption cooling system with silica gel as the adsorbent and water as the adsorbate was experimentally studied. To reduce the manufacturing costs and simplify the construction of the adsorption chiller, a vacuum tank was designed to contain the adsorption bed and evaporator/condenser. Flat-tube type heat exchangers were used for adsorption beds in order to increase the heat transfer area and improve the heat transfer ability between the adsorbent and heat exchanger fins. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 4.3 kW and a coefficient of performance (COP) for cooling of 0.45 can be achieved. It has provided a specific cooling power (SCP) of about 176 W/(kg adsorbent). With lower hot water flow rates, a higher COP of 0.53 can be achieved.     

Reducing cold-start emission from internal combustion engines by means of thermal energy storage system

Increasing environmental pollutions is an important problem appearing at cold start of internal combustion engines. Developments of new devices that solve this problem are an extremely urgent need especially for cold regions. In this study, a developed experimental sample of thermal energy storage system (TESS) for pre-heating of internal combustion engines has been designed and tested. The development thermal energy storage device (TESD) works on the effect of absorption and rejection of heat during the solid–liquid phase change of heat storage material (Na₂SO₄ · 10H₂O). The TESS has been applied to a gasoline engine at 2 °C temperature and 1 atm pressure. Charging and discharging time of the TESD are about 500 and 600 s, respectively and temperature of engine is increased 17.4 °C averagely with pre-heating. Maximum thermal efficiency of the TESS system is 57.5 % after 12 h waiting duration. CO and HC emissions decrease about 64% and 15%, respectively, with effect of pre-heating engine at cold start and warming-up period.     

Experimental evaluation of a low-power direct air-cooled double-effect LiBr–H2O absorption prototype

This paper describes a new small air-cooled double-effect LiBr–H₂O absorption prototype directly powered by fuel and discusses the experimental findings for some tests carried out in Madrid in 2007, with natural gas as energy source. The prototype, which has been designed to supply 7 kW of cooling power, was able to chill water up to 7–18 °C under extreme outdoor temperatures. A new flat-sheet adiabatic absorber was used allowing it to operate at outdoor temperatures about 45 °C without any sign of crystallization. A mean daily coefficient of performance (COP) of about 1.05 was obtained. Since this absorption machine does not need cooling tower, there is neither water consumption nor Legionella pollution. Moreover, it is a quite compact unit. The ratio of cooling power over volume is about 6.0 kW/m³, while for the only air-cooled absorption chiller, Rotartica 045v, in the marked until 2009 this ratio is 4 kW/m³. When comparing with electric chillers presently on the market, this prototype was found to have a cooling cost approximately 15.9% higher and an environmental impact 16.7% lower. The absorption prototype is a more environmentally friendly solution as it does not emit fluorinated refrigerants.     

Hydrogen production with integrated microchannel fuel processor for portable fuel cell systems

An integrated microchannel methanol processor was developed by assembling unit reactors, which were fabricated by stacking and bonding microchannel patterned stainless steel plates, including fuel vaporizer, heat exchanger, catalytic combustor and steam reformer. Commercially available Cu/ZnO/Al₂O₃ catalyst was coated inside the microchannel of the unit reactor for steam reforming. Pt/Al₂O₃ pellets prepared by ‘incipient wetness’ were filled in the cavity reactor for catalytic combustion. Those unit reactors were integrated to develop the fuel processor and operated at different reaction conditions to optimize the reactor performance, including methanol steam reformer and methanol catalytic combustor. The optimized fuel processor has the dimensions of 60 mm × 40 mm × 30 mm, and produced 450sccm reformed gas containing 73.3% H₂, 24.5% CO₂ and 2.2% CO at 230–260 °C which can produce power output of 59 Wt.     

Heat transfer and single-phase flow in internally grooved tubes

This study investigates heat transfer and flow characteristics of water flowing through horizontal internally grooved tubes. The test tubes consisted of one smooth tube, one straight grooved tube, and four grooved tubes with different pitches. All test tubes were made from type 304 stainless steel. The length and inner diameter of the test tube were 2 m and 7.1 mm, respectively. Water was used as working fluid, heated by DC power supply under constant heat flux condition. The test runs were performed at average fluid temperature of 25 °C, heat flux of 3.5 kW/m², and Reynolds number range from 4000 to 10,000. The effect of grooved pitch on heat transfer and pressure drop was also investigated. The performance of the grooved tubes was discussed in terms of thermal enhancement factor. The results showed that the thermal enhancement factor obtained from groove tubes is about 1.4 to 2.2 for a pitch of 0.5 in.; 1.1 to 1.3 for pitches of 8, 10, and 12 in., respectively; and 0.8 to 0.9 for a straight groove.     

The effects of geometrical parameters of a corrugated channel with in out-of-phase arrangement

Numerical investigations of forced turbulent convective flow and heat transfer in a corrugated channel of plate heat exchanger are carried out. The continuity, momentum and energy equations were solved by means of a finite volume method (FVM). The top and bottom walls of the corrugated channel are heated at constant heat flux boundary conditions. The effects of geometrical parameters of corrugated tilt angles, channel heights and wavy heights using water as a working fluid on the thermal and flow fields as well as on the performance of evaluation criterion are studied. The corrugated channel with three different corrugated tilt angles of 20°, 40° and 60° with different channel heights of 12.5, 15 and 17.5 mm and different wavy heights of 2.5, 3.5 and 4.5 mm are tested. This investigation covers Reynolds number and heat flux in the range of 8000–20,000 and 0.4–6 kW/m², respectively. The numerical results indicate that the wavy angle of 60° and wavy height of 2.5 mm with channel height of 17.5 mm are the optimum parameters and they have a significant effect on the heat transfer enhancement. It is found that using wavy channel is a suitable method to increase the thermal performance and getting higher compactness of the heat exchanger.     

Design and performance of a solar-powered heating and cooling system using silica gel/water adsorption chiller

In this paper, a solar-powered compound system for heating and cooling was designed and constructed in a golf course in Taiwan. An integrated, two-bed, closed-type adsorption chiller was developed in the Industrial Technology Research Institute in Taiwan. Plate fin and tube heat exchangers were adopted as an adsorber and evaporator/condenser. Some test runs have been conducted in the laboratory. Under the test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP (coefficient of performance for cooling) of 0.37 can be achieved. It has provided a SCP (specific cooling power) of about 72 W/(kg adsorbent). Some field tests have been performed from July to October 2006 for providing air-conditioning and hot water. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily average COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW. A typical daily operation shows that the efficiency of the solar heating system, the adsorption cooling and the entirely solar cooling system is 28.4%, 45.2%, and 12.8%, respectively.     

Single-phase heat transfer in the high temperature multiple porous insulation

An experimental study of steady state flow and heat transfer has been conducted for the multiple plate porous insulation used in the reactor pressure vessels of ‘Magnox’ nuclear power stations. The insulation pack studied, consisting of seven dimpled stainless steel sheets and six plane stainless steel sheets, was of the type installed in the Sizewell A plant. A large scale experimental test facility, based on the guarded hot plate method, was used for measuring the effective thermal conductivity of Magnox reactor pressure vessel insulation, which consists of alternate layers of plain steel foil and dimpled foil. The measurements were made both with the fluid within the insulation pack nominally stationary and with an imposed flow through it, simulating leakage through the insulation pack. The experimental conditions corresponded to a heat flux of 75–1000 W/m², fluid pressures of atmospheric to 5 bar gauge, pack orientations in range of 0°–45° relative to the horizontal, leakage velocities ranging from 0.05 m/s to 0.20 m/s and inlet air bulk temperatures ranging from 18 °C to 290 °C. Local values of effective thermal conductivity of 0.04–0.23 W/m K were obtained for the above experimental conditions. The heat transfer modes in the insulation pack were conduction through the contacting metallic foils, thermal radiation across the gas gaps, and conduction and convection in the air. The effective thermal conductivity of the porous insulation increased with increasing air pressure, inclination angle, and air velocity. Buoyancy effects increased with increasing inclination angle and air pressure.     

Effect of gamma ray irradiation on thermal and electrochemical properties of polyethylene separator for Li ion batteries

Polyethylene separator is irradiated by gamma ray to investigate the effect on electrochemical and thermal properties of polyethylene separators. Applied irradiation dose was 50, 100, 150 and 200 kGy. It is observed that the melt integrity temperature is increased as the higher dose of irradiation is applied. In case of non-irradiated PE separator, shut-down temperature is about 133 °C and melt integrity temperature is 146 °C. On the other hand, the separator with irradiation dose of 200 kGy, shut-down temperature is about 136 °C and melt integrity temperature is about 166 °C. Thermal shrinkage test is performed at 150 and 120 °C for 1 h each. It is observed that the irradiated separator has better thermal shrinkage resistance than the non-irradiated one because of the cross-link of polyethylene formed by gamma ray irradiation. Therefore, it can be concluded that the cross-link of polyethylene formed by gamma ray irradiation is strongly effective against thermal shrinkage. The electrochemical test is performed at high rate using the irradiated separator and the non-irradiated one to investigate any trade-off. The high rate discharge characteristic of the irradiated separator is better than that of the non-irradiated one. FTIR spectral analysis of the irradiated and non-irradiated separators is performed and ionic conductivity was measured to understand the reason.