E. Ienei 1*, M. Voinea1, R. Lates1, L. Isac1, I. Visa1, A. Duta1

1 The Centre: Product Design for Sustainable Development, Transilvania University, Eroilor 29, 500036,
Brasov, Romania, Tel. 004-0740054297, Fax 004-0268410525,

*email: e. purghel@unitbv. ro

Abstract

Selective surfaces are required to improve the solar collectors’ efficiency, thus the conversion of solar radiation into thermal energy. Performing coatings should have high solar absorption coefficient (as>0.9) in the wavelength range of 0.3-2.5 pm and low emittance (s< 0.1) for wavelength higher than 2.5 pm; high durability absorbers must be obtained using low amounts of chemicals, as thin films deposited on large surfaces, in environment-friendly processes with low energy consumption.

The aim of this paper is to test the efficiency of two new thin film composite absorbers deposited on aluminium and copper substrates. The absorbers, based on either copper oxides/nickel oxides or alumina/nickel oxides and coated with an antireflective layer: titania, were integrated into a laboratory collector and their efficiency was measured for different incidence angles. The results proved that their efficiencies are comparable, with better results for the absorber with copper substrate. The optimum incident angle (from the horizontal plane) for collectors with this type of absorbers is 10-30 degree.

Keywords: flat plate collector, solar absorbers testing, spray pyrolysis, incident angle.

1. Introduction

The climate changes and the increasing energy consumption are one of the mankind major problems. Each product needs to be re-designed as to have a minimum environment impact, on all its life cycle steps. Not only the payback time has to be considered in the product development, but also the energy payback time.

In a sustainable future, buildings with low energy consumption must be developed. This can only be achieved by implementing saving/renewable energy systems. The solar collectors can be efficiently used for heating/cooling applications. Still, technical improvements have to be made in order to identify more cost efficient solutions considering the embodied energy and materials. The solar absorber, the active part of a photo-thermal device, is the most expensive component of the system; therefore new methods should be developed to decrease the cost, without a major effect on its performance. Generally, the absorber plate is made of a highly thermal conductivity metal (Al, Cu) coated with a spectrally selective surface, with maximum absorption across the solar spectrum and minimum emission in the infrared region, [1-4]. Absorbers must be obtained using a low amount of chemicals (non-toxic), on large surfaces in order to minimize the energy consumption and to protect the environment.

The first selective coatings were based on black paints [5]. Because the temperature of the absorber surface is above ambient temperature, the surface re-radiates some of the heat back to the

surroundings. This loss mechanism is a function of the surface emittance of the low-temperature, long-wavelength (infrared) radiation. Therefore, these coatings have high absorption in the short — wavelength radiation, but also high long wavelength radiation loss from the surface [6]. On the other hand, the nowadays state of art selective coatings are rather expensive due to the complex deposition methods and/or the obtaining process are involving toxic (by) products.

The aim of this paper is to test the efficiency of two new, low cost, black nickel based absorbers deposited on aluminium and copper substrates. The solar absorbers were integrated into a laboratory flat plate collector and their functional parameters were measured for different incidence angles. The results, in terms of heat and efficiency are discussed in this paper.

2. Experimental