Characterization of a Water Heating System Using Solar Collector With Conical Concentrator

This study aimed to evaluate a solar water heating system for using on residences, using a solar collector with conical concentrator. The principle of light concentration in a solar collector with conical concentrator is the capture and reflection of solar radiation in the center of a tapered concentrator with internal reflective faces. The area of concentration of solar energy is occupied by a receiver with material of high thermal conductivity, properly isolated by transparent surfaces, to form the greenhouse effect, where the thermal energy is transferred to a working fluid. The characterization of the system was done through field tests to determine the efficiency in the water heating. The tests were performed considering different scenarios, which varied according to the heating system (passive and active with different water flow) and solar tracking (manual adjustment and stationary). The results showed that the scenarios with solar tracking presented an average efficiency of 12.63%, which was more efficient than those presented by the fixed orientation, which was 11.44%. Besides that, it was verified that the active solar heating systems were more efficient than the passive ones.


Introduction
The increase of the energy demand has encouraged researches into energy generation technologies by renewable sources with more efficient applications in the effort to reduce the supply of non-renewable fuels.It is very important to use agriculture science to give support to small farmers.The study of different water systems can contribute to foment agribusiness in small rural areas.
In this context, the solar energy is one of the main sources of energy for the sustainable development, and has an enormous potential in Brazilian territory, as it has an incidence considered relatively high (Luiz, 2013).
The solar energy, directly or indirectly transformed into thermal energy, can be considered a good alternative for heating of water for various purposes, presenting many advantages with regard to the conventional energy sources.Efforts to the harnessing of this energy, as well as its large-scale application, have made solar heating systems cheaper and more efficient demonstrating to be a viable and competitive alternative (Aldabó, 2002;Mourão, 2007).

Solar Geometry and Concentrators
The solar concentrators can be applied for heating of air, water and/or generation of electric power.They can be hybrid systems that concentrate the solar energy into photovoltaic cells, being the excessive heat transferred to fluids such as air and water (Zahedi, 2011).For the correct dimensioning and using of these systems, it is necessary to know the angles of the solar geometry and the types of collectors, as shown in next items.

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The efficie systems w efficiencie According than the pa drastically conical so min -2 ) and highlight t A12S > P efficiency efficient.It is noteworthy that the efficiency in the systems with fixed orientation showed smaller variations over the time, compared to the systems operated with solar tracker.The average values of efficiency presented in Figure 30 were 17.69; 10.05 and 6.59% and presented maximum efficiency of 37.54; 14.82 and 12.38% respectively for the A24E, A12E and PE systems.
However, in relation to the efficiency of all the trailed scenarios, it is verified that the systems with solar tracker showed higher values of average and maximum efficiency in relation to the fixed guidance systems, except the A24S system.
Knowing that the efficiency is a relation between useful energy and accumulated solar radiation, it can be noticed in Figures 27 and 28, that the solar radiation harnessed by the A24S system is larger than that harnessed by the A24E system.However, the useful energy gains, although they have the same tendency (A24S > A24E), are not as expressive as the harnessed solar radiation.
The heat losses that occur in the evaluated heating systems, responsible for the low efficiencies, are due to the lack of adequate thermal isolation in the reservoir and in the pipes.
A study was developed with a heating system working with different mass flow, in order to obtain the system efficiency (Pereira et al., 2006).It was developed a solar air heater through least-squares support vector machines, and obtained an efficiency that varied from 0.09 to 0.17, for an air mass flow rate of 0.03 kg s -1 , and an efficiency that varied from 0.35 to 0.55, for an air mass flow rate of 0.05 kg s -1 .
An experimental investigation was developed for analyzing the thermal performance of a double-flow solar air heater having aluminum cans (Esen et al., 2009).The efficiency obtained for 0.05 kg s -1 air mass flow varied from 0.3 to 0.8, and for 0.03 kg s -1 , varied from 0.2 to 0.6.

Conclusions
The solar collector developed in this work has proved technically feasible for water heating, reaching considerable temperatures for the use in residences.
The results showed that the scenarios with solar tracking presented an average efficiency of 12.63%, which was more efficient than those presented by the fixed orientation, which was 11.44%.
In general terms, considering the variables analyzed and the results of the tests performed, it can be concluded that: i) The geometry of the concentrator set and absorber was effective to provide thermal energy gains for the fluid, even in periods where the concentrator capture plane is not perpendicular to the direction of the flow of the incident solar rays; ii) Solar water heating systems operated with solar tracker show more efficiency in relation to the stationary systems; iii) Active solar heating systems are more efficient than the passive systems operated by thermosyphon; iv) It can be increased the efficiency of active solar heating systems by raising the mass flow to a critical value.
For future studies, it is recommended an evaluation of conical solar collector efficiency with automatic solar tracker, a study of the conic concentrator geometry for different angles, an analysis of the thermal efficiency behavior for a wider range of water flows, and a simulation of the optical behavior for stationary solar concentrators.