4. Thermoelectric refrigeration unit applications

off-electricity times [46, 47]. Two prototypes of thermoelectric refrigerator are described in [48], one with finned heat sink and the other one with a finned heat sink integrated in an aluminum thermosiphon in which phase change occurs. The thermosiphon depends on the specific latent heat at the phase change from vapour state to liquid state, useful to disperse the heat efficiently to the environment. The results of the experimental heat sink optimization demonstrated that the thermal resistance between the hot side of the TEC and the environment reduced with about 23.8% at 293 K environment temperature and 51.4% at 308 K, with respect to a commercial finned heat sink, and between 13.8% and 45% with respect to an optimized finned heat sink. Much more, the COP of this prototype is 26% at ambient temperature of 293 K, achieving

3.4. Optimization of the internal temperature controller of the insulated compartment

between higher and lower setpoints of the internal temperature [49].

The operating conditions of a thermoelectric refrigerator depend on parameters as environment temperature, humidity, lower setpoint of the internal temperature and difference

In vapour-compression refrigerators, the internal temperature control inside the insulated compartment is generally inaccurate due to the multitude of start and stop cycles made by the compressor, leading to a temperature variation bigger than 8C, with a negative effect on the quality of the food and on the conservation of the perishable food [50, 51]. This represents a drawback for these refrigerators compared with the thermoelectric refrigerators in which there are no start and stop cycles and the supply voltage gradually increases. However, overall the thermoelectric refrigerators are not competitive with vapour-compression refrigerators in

Most of the thermoelectric refrigerators have on/off control systems for the internal temperature. This control is critical in the period in which the TEC is switched off, because in this period, the heat stored in the heat sink connected to the hot terminal returns into the refrigerator compartment; in this way, the power consumption of the refrigerator increases and the

Vian and Astrain [50] carried out a study of the total power consumption on a hybrid thermoelectric system (with vapour freezer and refrigerator compartments and a thermoelectric compartment) at ambient temperature of 25C. To optimize the system, a thermal bridge (aluminium slab) was used between the freezer compartment and the thermoelectric compartment. This thermal bridge was useful to transfer the heat flow rate from the thermoelectric compartment to the freezer in order to maintain a constant temperature of 0C inside the thermoelectric compartment. The power consumption in these environmental conditions for each compartment was 0.67 kWh/day for the refrigerator, 0.58 kWh/day for the freezer and 0.2 kWh/day for the thermoelectric system. The results demonstrated that the total electric power consumption reduced from 63.3 W to 49.9 W (20% improvement) due to the thermal bridge. If the environmental conditions are modified (e.g., a rise of the temperature at 30C), the total power consumption for this unit rises by 30%. Further studies showed that the thermoelectric refrigerator works in any operating condition, but the utilization of on/off

36.5% improvement at 303 K.

238 Bringing Thermoelectricity into Reality

terms of COP [10, 11, 48].

COP decreases [11].

In spite of their relatively low efficiency with respect to other refrigeration technologies, the TEC technologies are experiencing a period of development, with subsequent efficiency improvement and reduction of the manufacturing costs [52]. One of the drivers that have increased the interest in the development and use of TECs as refrigerators is the absence of environmental pollution in the TEC operation, in particular, the absence of chlorofluorocarbon (CFC) issues. The current trends towards replacement of CFCs consider good solutions with low global warming potential (GWP) using natural refrigerants like CO2 used at pressures much higher than traditional refrigerants [6, 53]. Further drivers to increase the TEC applications depend on positive aspects of the TECs such as low noise, possibility of operation in different positions, absence of mechanical vibrations, ease of transportation and possibility to obtain accurate temperature control.

Today, thermoelectric refrigerators are the most significant applications at the commercial level [17, 40]. In addition to domestic refrigerators [10, 54, 60], other applications have been developed for food-related services, such as portable refrigerators [55–57], food expositors, refrigerators mounted on vehicles for perishable food transportation as well as low-power refrigerators for minibar, hotel room, offices, boats and aircraft services [8]. Further applications are available for the medical sector (vaccine transportation and instruments for blood coagulators, dew point sensors and others), for the military sector and for scientific devices subject to precise temperature control [58]. In addition, thermoelectric systems are found in the automobile industry for air conditioning or car-seat coolers [59] and in different applications to the microelectronics sector [60, 61].

Besides the applications mentioned above, the present trend towards the use of green energy raises the attention on the possibility of supplying the thermoelectric refrigerator through energy produced from renewable sources. The refrigerators powered by renewable sources may work in stand-alone of off-grid connection. To connect a thermoelectric refrigerator to the PV module in off-grid mode, the possibilities are [8]:

different capacities (from a few litres for medicine transportation to some hundreds of litres for food storage), temperature difference, type of heat sink, AC or DC voltage input, powering from electrical grid connection or PV and electrical power input. The performance of these

Thermoelectric Refrigeration Principles http://dx.doi.org/10.5772/intechopen.75439 241

Thermoelectric refrigeration solutions are gaining relevance because of a number of positive aspects, such as long duration, noiseless operation, limited maintenance needs, absence of flammable or toxic refrigerants, possibility of being used in different positions and in movable solutions as well as flexibility of usage through optimized control. This chapter has summarized the principles of thermoelectric refrigeration, by presenting the analytical formulations determining the heat flow rate, cooling capacity and COP of a TEC, illustrating the methods to enhance the TEC performance and indicating the current applications of thermoelectric refrigeration. The future improvement of the TEC performance, together with the operational flexibility of the TEC driven by appropriate control systems, will increase the variety of the applications of thermoelectric refrigeration in different contexts, from single units to their inclusion into integrated

Department of Electronics, Telecommunications and Energy, Valahia University of Targoviste,

[1] Zheng JC. Recent advances on thermoelectric materials. Frontiers of Physics in China.

[2] Sandoz-Rosado EJ. Investigation and Development of Advanced Models of Thermoelectric Generators for Power Generation Applications [thesis]. Rochester Institute of Technology, Rochester NY, USA; 2009. 82 p. Available from: http://scholarworks.rit.edu/theses

[3] Rowe DM. Handbook of Thermoelectrics. Introduction. Boca Raton, FL: CRC Press; 1995

[4] Lee HS. Thermal Design, Heat Sinks, Thermoelectrics, Heat Pipes, Compact Heat Exchangers, and Solar Cells. NJ: Wiley; 2010. 650 p. DOI: 10.1002/9780470949979

units is indicated with cooling capacity and COP.

Address all correspondence to: diana.enescu@valahia.ro

2008;3(3):269-279. DOI: 10.1007/s11467-008-0028-9

720 p. ISBN: 9780849301469

5. Conclusions

energy systems.

Author details

Targoviste, Romania

Diana Enescu

References


Solar-driven thermoelectric refrigerators are of two types, namely, PV-battery thermoelectric systems and PV-PCM thermoelectric systems. The performance of the PV-battery thermoelectric systems depends on the intensity of solar radiation and temperature difference at the hot and cold sides of the TEC. In the case of PV-PCM thermoelectric systems, the PV is directly connected to the TECs having PCMs fixed at the cold side to replace the battery. Thermal storages have generally restricted capacity, and to improve this in some applications, the thermoelectric units use PCM integrated with thermal diodes [62].

Table 1 presents the technical characteristics of some selected thermoelectric refrigeration units with data available from the literature. The selected cases represent various applications with


Table 1. Technical characteristics and performance of thermoelectric refrigeration units.

different capacities (from a few litres for medicine transportation to some hundreds of litres for food storage), temperature difference, type of heat sink, AC or DC voltage input, powering from electrical grid connection or PV and electrical power input. The performance of these units is indicated with cooling capacity and COP.
