Nanofluid solar collectors

Modelling the efficiency of a nanofluid-based direct absorption collector

Solar energy has the capacity to solve one of the most pressing needs facing modern society: namely the development of a sustainable and environmentally friendly energy source. The potential benefit of this decentralised and inexhaustible natural source is demonstrated by the fact that more solar energy strikes the Earth in one hour than the total energy consumption of humans per year. Moreover, the available solar power striking the earth’s surface at any one time is equal to 130 million 500 MW power plants. The current challenge is to develop a truly widespread, cost-effective system which efficiently converts solar energy and can compete with fossil fuel power generation.

Nanofluid-based direct absorption solar collectors (NDASCs) have the potential to harness solar energy significantly more efficiently than traditional collectors. Experimental research has shown that dispersing trace amounts of nanoparticles into a base fluid can significantly enhance the chemical, optical and thermophysical properties of the fluid. Nanofluids also demonstrate improved stability and rheological properties when compared with fluids containing microparticles. At present, NDASCs are not yet at an economically viable level of development. To achieve this goal requires further experimental and theoretical research to optimise the effectiveness of the nanofluid in converting solar to thermal energy.

The goal of our project was to use an analytical method to model the efficiency of a low to mid-temperature NDASC. The governing two-dimensional model consisted of a system of two coupled differential equations; a radiative transport equation (RTE) describing the attenuation of solar radiation in the nanofluid and an energy equation for the temperature of the nanofluid. In previous studies numerical simulations were typically required to solve the model, as the solution to the RTE involves a problematic wavelength-dependent integral for the heat source term in the energy equation. This integral cannot be evaluated directly. Our approach was to compute this integral numerically and then fit a power-law function to the numerical results. The main advantage of our method is that all of the wavelength-dependent optical properties can be retained in the model. We also included an approximation for the total system reflectance and absorptance. Our relatively simple approach allowed for an exact solution to the heat equation via the method of separation of variables. The resulting temperature solution was used to ascertain the efficiency of a collector containing a water-aluminium nanofluid. We investigated the collector performance subject to variation in system parameters such as the nanoparticle concentration, collector height, particle diameter and collector angle.

In line with previous numerical and experimental studies we observed that the collector efficiency depends strongly on nanoparticle concentration. Specifically, for small concentrations the efficiency rapidly increases. However, there is a concentration limit beyond which it becomes pointless to add more particles. Similarly, efficiency increases with collector height. Particle diameter and collector angle were shown to have little effect. In addition, the dimensionless form of the model highlighted that there were five controlling nondimensional groups, specifically two that describe the heat source, one describing the relative importance of conduction and advection and two describing the heat loss to the surroundings.

Figure taken from http://greenliving4live.com/2014/03/learn-everything-need-know-solar-energy/

Article written by Vincent Cregan (vcregan@crm.cat}