Evangelos Bellos

A review of concentrating solar thermal collectors with and without nanofluids

Solar concentrating solar thermal collectors are promising technologies for various applications which demand medium- and high-temperature levels. The objective of this work is to review the recent trends in the solar concentrating collectors and to give the emphasis on the performance enhancement methods which applied to the concentrating technologies. Optical and thermal enhancements methods are investigated for the following collector types: compound parabolic concentrator, parabolic trough collector, linear Fresnel reflector and solar dish concentrator. The emphasis is given to the utilization of nanofluids as working fluids because a lot of research has been focused on them in the last years. Moreover, the use of internal fins and inserts in the flow, the use of modified absorbers, as well as various optical design optimizations are included in this review. The final conclusions of this work clearly indicate the most effective enhancement methods in the concentrating solar collectors, as well as the future fields that have to be investigated.

Solar Energy Management Using Phase Change Materials Passive Systems in the Athens Area Buildings

A numerical procedure is developed for the management of solar energy in buildings using passive systems, which are based on Phase Change Materials (PCM). For the simulation of the phase change process, the concept of effective thermal capacity function is used, which is determined experimentally and then generalized using triangular functions. The developed procedure is applied along the typical year in the Athens area buildings for studying the effect of main PCM parameters, including the phase change temperature range, the phase change heat and the thickness, location and way of embodiment of PCM within buildings elements. It is found that energy savings up to 33% may be obtained along the Athens typical year by placing at the proper building location PCM layers with the right thermal properties. The conclusions of the study may be used, apart from the Athens area, to regions of similar climate and moreover, because of their generality, most of them may be useful to any climatic conditions.

Thermal and exergetic evaluation of parabolic trough collectors with finned absorbers operating with air

The objective of this work is to evaluate energetically and exergetically the use of internal longitudinal fins in parabolic trough collectors operating with air. Nine different finned absorbers are compared with the smooth absorber for various inlet temperatures up to 500℃. More specifically, the use of 4, 8, and 16 fins with lengths 5 mm, 10 mm, and 15 mm are examined. The simulation tool is Solidworks Flow Simulation and the examined parabolic trough collector is the Eurotrough module. According to the final results, the global maximum exergetic efficiency is 43.65% and it is achieved for 4 fins with 15 mm length, while the inlet temperature of the air is equal to 350℃ and the thermal efficiency is 67.98%. Moreover, in the cases of 8 and 16 fins, the optimum lengths exergetically are 10 mm and 5 mm respectively, a fact that proves the reverse relationship between the number of fins and the fin length. Finally, it is also important to be stated that greater number of fins and higher fin length not only lead to higher thermal performance but also to higher pressure losses; two factors are taken into consideration in the exergetic performance, which is the main evaluation index of this study.

Comparison of Heating and Cooling Loads of a Typical Building with TRNSYS and eQUEST

The energy performance of buildings can be estimated using various software which use different models for this simulation. It is important to know the value of heating and cooling loads of buildings in order to design the optimum system in every case for minimize the cost and the fuel or electricity consumption. In this study, the loads of a typical building are calculated by two different software, TRNSYS and eQUEST. These programs have different strategy in the loads calculation fact that makes this comparison important in order to validate the results.

Evaluation of a solar driven trigeneration system with conventional and new criteria

ABSTRACT Solar-driven trigeneration systems are able to cover all the building energy needs in heating, cooling and electricity using only the solar energy. The objective of this paper is to evaluate a solar trigeneration system under different criteria in order to make clear that there are numerous factors which have to be taken into consideration in the trigeneration system design. The examined system consists of parabolic trough solar collectors, a storage tank, an organic Rankine cycle and an absorption heat pump. The system is evaluated using different evaluation criteria which are associated with the energy, the exergy and the financial performance of the system. Moreover, a new criterion which takes into account the building energy needs is introduced and is investigated in different scenarios. The final results of this work clearly indicate that the optimisation of the solar driven trigeneration systems is depended on the desired design conditions and goals. Abbreviations: COP, Coefficient of performance; ECO, Economizer; EES, Engineering Equation Solver; EVAP, Evaporator; ORC, Organic Rankine Cycle; PTC, Parabolic trough collector; SC, Scenario

Modelling of a Solar Assisted Floor Heating System with TRNSYS

Energy consumption in buildings for heating purposes consists of a great percentage of the total building energy consumption. The increasing rate of electricity cost leads our society to use different and more efficient ways for covering the heating loads. The utilization of solar energy is an efficient way to cover the heating needs with a green and low cost way. In this study a solar assisted floor heating system is modelled and analyzed with the commercial software TRNSYS. The solar flat plate collectors operate during the day capturing the solar energy which is stored in the storage tank by increasing the water temperature. The hot water flows inside the tubes under the floor of the house when the inside temperature is under the desired level. An auxiliary heater gives extra energy in order to keep the desired thermal comfort conditions inside the building. Also a controlled mix between the water building flows is made in order to have the desired temperature in the inlet of the building.