Review of solar, heat pipe and thermoelectric hybrid systems for power generation and heating

Abstract

Abstract Solar energy is a renewable energy heat source freely and widely available everywhere throughout the year.Heat pipes are veryeffective and passive heat transfer devices. A solar heat pipe collector performs well at hightemperatures. Thermoelectricity could be utilized for power generation and provide cooling and heating. Thecombination of a solar heat pipe collector with thermoelectric modules could provide a very useful device forsimultaneous power generation and hot water heating. Such hybrid systems could offer small, mobile,transportable and off-grid power and heating systems for small-scale industry or domestic applications. Thispaper reviews some of theworks conductedon the solar/heat pipe/thermoelectric hybridsystem. Keywords: solar; heat pipe; thermoelectric; power generation; water heating Received 20 November 2014; revised 10 June 2015; accepted 22 July 2015 1 INTRODUCTION Solar energy is a renewable energy heat source freely and widelyavailable everywhere worldwide and throughout the year. Solarapplications can be classified under the headings of solar thermalor solar photovoltaic (PV). Numerous texts are available on thesubjectofsolarenergy[1,2].Heat pipes (HPs) are very effective and passive heat transferdevices that are capable of transferring large amounts of heatthrough long distances with small temperature differencesbetween the heat source and the heat sink. A HP consists of asealed copper pipe, vacuumed and filled with acertain amount ofworking fluid. There are three distinct sections in the pipe. Theevaporator section is separated from the condenser section by ashort adiabatic section. Heat applied over the evaporator sectionof the pipe causes the fluid to boil internally and vaporize,picking up latent heat of vaporization. The vapor travels insidethe sealed pipe to the condenser section where it condenses,giving up its latent heat of condensation. The condensate is thentransported back to the evaporator section either by gravityor viainternally incorporated wicks. Heat is thus transferred from theevaporator to the condenser section of the pipe. ComprehensiveoverviewsofHPscanbefoundinReferences[3–6].Thermoelectric (TE) is the direct conversion of a tempera-ture difference imposed between the junctions of twodissimilar materials to electricity. This is known as the Seebeckeffect and attributed to Thomas Seebeck in the nineteenthcentury.ItsapplicationissometimesreferredtoasTEpowergeneration (TEG). Later, Peltier showed that the converse istrue. By imposing avoltage across or passing a current throughtwo dissimilar materials, a temperature gradient is createdbetween them. A hot surface in contact with the cold junctionoftheTEwillbecooleddownandvice versa, a cold surfacein contact with the hot junction will be heated up. This effectcan be used for TE cooler (TEC) heat pump heating andcooling applications. Some useful references could be found inReferences [7,8].A typical solar/HP/TE (SHPTE) hybrid system consists of asolar HP collector with a TE module connected at the condenserend as illustrated in Figure 1a. The TE module is cooled with awater-cooled jacket or by an air-cooled fin heat sink. In order toobtain high temperatures, the solar collector is normally of theevacuated tube HP solar collector (ETHPSC) type. An end viewof a typical ETHPSC unit is shown in Figure 1b. The glass enve-lope consists of two concentric glass tubes with a selective surfacecoating applied on the outer surface of the inner tube. The tubesare evacuated and sealed at the ends. This reduces heat loss to theambient and results in a high-performance unit. A smaller diam-eter copper HP is suspended in the center of the evacuated glasstubes and held in place by a pre-formed aluminum fin wrapped