Aliakbar Akbarzadeh

Heat pipe-based cooling systems for photovoltaic cells under concentrated solar radiation

Concentrating solar radiation onto photovoltaic solar cells does not generally produce a uniform distribution of solar radiation at the surface of the cells. In this work a unique profile for the reflecting surfaces has been developed such that the solar cells are evenly illuminated under any degree of concentration. Also introduced is a passive method based on thermosyphons which can effectively cool the solar cells under concentrated light. A prototype of an east-west trough solar concentrator using the profile developed for the reflecting surface, and incorporating a thermosyphon cooling system for the photovoltaic cells, has been manufactured and successfully tested. The results are reported.

Effect of wick characteristics on the thermal performance of the miniature loop heat pipe

Two phase heat transfer devices based on the miniature version of loop heat pipe (LHP) can provide very promising cooling solutions for the compact electronic devices due to their high heat flux management capability and long distance heat transfer with minimal temperature losses. This paper discusses the effect of the wick properties on the heat transfer characteristics of the miniature LHP. The miniature model of the LHP with disk-shaped evaporator, 10 mm thick and 30 mm disk diameter, was designed using copper containment vessel and water as the working fluid, which is the most acceptable combination in electronic cooling applications. In the investigation, wick structures with different physical properties including thermal conductivity, pore radius, porosity, and permeability and with different structural topology including monoporous or biporous evaporating face were used. It was experimentally observed that copper wicks are able to provide superior thermal performance than nickel wicks, particularly for low to moderate heat loads due to their low heat conducting resistance. With monoporous copper wick, maximum evaporator heat transfer coefficient (hev) of 26,270 W/m2 K and evaporator thermal resistance (Rev) of 0.06–0.10°C/W were achieved. For monoporous nickel wick, the corresponding values were 20,700 W/m2 K for hev and 0.08–0.21°C/W for Rev. Capillary structure with smaller pore size, high porosity, and high permeability showed better heat transfer characteristics due to sufficient capillary pumping capability, low heat leaks from evaporator to compensation chamber and larger surface area to volume ratio for heat exchange. In addition to this, biporous copper wick structure showed much higher heat transfer coefficient of 83,787 W/m2 K than monoporous copper wick due to improved evaporative heat transfer at wick wall interface and separated liquid and vapor flow pores. The present work was able to classify the importance of the wick properties in the improvement of the thermal characteristics for miniature loop heat pipes.

Application of heat pipe heat exchangers to humidity control in air-conditioning systems

Relative humidity control in air-conditioning systems is an important aspect of the maintenance of good indoor air quality. However, considerable reheat energy is needed to control relative humidity using conventional reheating coils. For improved energy efficiency a thermosyphon heat pipe heat exchanger is under investigation using a Hilton Air-Conditioning Laboratory Unit in RMIT. The study involves aspects of the heat exchanger, including temperature effectiveness, capability of energy recovery and relative humidity control of delivered air. It is observed that this type of heat exchanger can be an advantageous replacement for conventional reheat coils, resulting in energy saving and enhancing the cooling capability of the cooling coils with little or no external energy needed.

The design, construction, and initial operation of a closed-cycle, salt-gradient solar pond

Abstract In operation of a closed-cycle salt-gradient solar pond (CCSGSP) system, fresh or low salinity water is supplied at the surface of the solar pond (SP) as make-up for evaporation losses as well as for surface washing. In the present investigation the surface water is flushed to an evaporation pond (EP) and concentrated for reinjection at the bottom of the SP. A 20 m2 SP incorporating an EP for concentrating brine, has been established. Theoretical modelling of the CCSGSP is presented. Results from the initial operation of the SP show that wind action and convective mixing caused some erosion of the gradient layer thereby increasing the surface layer thickness. Salt flux to the surface was found to be approximately 19 kg/m2 per year. Sodium hypochlorite solution proved successful as shock treatment during severe algal bloom. The result of acidification was less promising in maintaining pond clarity. Occasional addition of alum helped in settling some of the suspended particulates in the pond.

Novel design of a miniature loop heat pipe evaporator for electronic cooling

Miniature loop heat pipes (mLHPs) are coming up with a promising solution for the thermal management of futuristic electronics systems. In order to implement these devices inside compact electronics, their evaporator has to be developed with small thickness while preserving the unique thermal characteristics and physical concept of the loop scheme. This paper specifically addresses the design and testing of a mLHP with a flat evaporator only 5 mm thick for the cooling of high performance microprocessors for electronic devices.

Towards the design of low maintenance salinity gradient solar ponds

Applications of simple methods to reduce the maintenance of a small solar pond are discussed in this paper. It was found that floating rings along with continuous surface flushing could effectively control and maintain a relatively thin upper convective layer. A novel system of salt replenishment (a salt charger) is introduced. It is shown that the application of the proposed system is capable of controlling the position of the lower interface. Criteria governing the design of a salt-charger for a salinity-gradient solar pond are developed theoretically and verified experimentally. The design procedures are presented. Experiences relating to the utilization of brine shrimps to improve the transparency of the pond are described.

Formulation and analysis of the heat pipe turbine for production of power from renewable sources

Abstract The heat pipe turbine or thermosyphon Rankine engine is a new concept for power generation using solar, geothermal or other available low grade heat sources. The basis of the engine is the thermosyphon cycle, with its excellent heat and mass transfer characteristics, modified to incorporate a turbine in the adiabatic region. The basic configuration is a closed vertical cylinder functioning as an evaporator, an insulated section and a condenser. The turbine is placed in the upper end between the insulated section and condenser section, and a plate is installed to separate the high pressure region from the low pressure region in the condenser. Conversion of enthalpy to kinetic energy is achieved through the nozzles. The mechanical energy developed by the turbine can be converted to electrical energy by direct coupling to an electrical generator. This paper describes the development of the heat pipe turbine from concept to reality, a series of development steps taken to optimise the design and manufacture. Also in this paper, attempts have been made to provide relationships for the developed power in terms of the geometric and thermodynamic parameters and to discuss limitations on the efficiencies of these turbines.

Design, manufacture and testing of a closed cycle thermosyphon rankine engine

Abstract The Thermosyphon Rankine Engine (TSR) is a recent concept for power generation using solar or other available low grade heat sources. The basis of the engine is the modification of a heat pipe, with its excellent heat and mass transfer characteristics, to incorporate a turbine, thereby making the system into a Rankine Cycle Engine. The TSR is directed towards power production from solar ponds, geothermal energy and heat produced by solar collectors, as well as for waste heat utilisation for electrical power generation. A theoretical formulation and results from experiments on prototype units are presented. Based on the results, it is concluded that the TSR engine may play an important role for conversion into electrical energy of thermal energy produced by conventional solar collectors, geothermal sources and waste heat.

Characteristics of a gravity-assisted heat pipe-based heat exchanger

Abstract An experimental study of the performance of an air-to-air thermosyphon-based heat exchanger utilizing R-22 as the working fluid has been carried out to investigate its behavior under different operating conditions. A test installation has been developed to model a variety of HVAC real life applications. The results reported in this article describe the influence of various parameters such as: supply and exhaust air stream mass flow rates, stream temperatures and exhaust stream moisture content on the effectiveness of the heat exchangers. Heat exchanger heat flow hysteresis has been recognized. Some optimization criteria are presented.

Characteristics of two-phase closed thermosiphons for medium temperature heat recovery applications

Application of two-phase closed thermosiphons to heat recovery systems has led the authors to investigate the performance of thermosiphons at medium temperatures. Two-phase closed thermosiphons working under various conditions have been tested and their thermal performance has been measured for mean evaporator wall temperatures between 100°C and 250°C. A description of the design and construction of the test facility is included. Aspects of safety of container materials have been investigated for water as the working fluid. It was found that copper-nickel alloys and carbon-manganese stainless steel are suitable container materials for the range of temperatures considered. The critical heat flux and dry-out limit were observed in experiments with a 13.2 mm diameter thermosiphon. Boiling heat transfer phenomena and overall thermal conductance have been experimentally investigated. It was found that an increase in thermosiphon diameter changes the boiling mechanism from saturated film boiling to nucleate boiling. The test results show a good agreement with published correlation criteria. Further experiments are needed to determine the optimum pipe diameter for the applications under consideration.