Study on the intermittence of certain energy sources based on a nonendoreversible power plant model

In a previous work, we studied the thermoeconomics of a nonendoreversible simplified thermal power plant model (the so-called Novikov engine) in terms of the maximization of an objective function defined as the quotient of characteristic functions (power output and ecological function) and the total costs involved in the plant performance. In our thermoeconomic study, we used different heat transfer laws: the so called Newton's law of cooling, the Stefan-Boltzmann radiation's law, the Dulong-Petit's law of cooling and another linear phenomenological heat transfer law. In this work, we find some analytical expressions for the optimal efficiencies in terms of the fractional fuel cost and we show that the optimal efficiency at maximum ecological function regime is somewhere between the maximum-power efficiency (Curzon-Ahlborn efficiency) and the maximum-efficiency (non-endorreversible Carnot efficiency). Besides, we show that under maximum ecological conditions the wasted energy towards the environment is diminished in comparison with the wasted energy under maximum power conditions. From the thermoeconomic optimization of the power plant model we obtain the optimal efficiencies in terms of the fractional fuel cost (defined as the ratio of the fuel cost and the total cost involved in the plant performance). We assume that the intermittence of renewable energy sources can be taken into account by the non-endoreversible parameter, which diminishes both power output and the optimal efficiency. Thus, in terms of the optimal efficiencies for both maximum power output and maximum ecological function conditions we can calculate the fractional fuel cost in terms of the irreversibilities which are associated to the reductions of both the power output and the optimal efficiencies of the plants. If we use some data reported by De Vos for the case of solar photovoltaic plants in the analytical expressions for the fractional fuel cost, we obtain that the fuel cost is three times the investment cost when the power plant operates at maximum power conditions. On the other hand, when the power plant operates at maximum ecological function conditions, we obtain that the fuel cost is approximately 0.2 times the investment cost. Therefore, by means of the proposed approach we can treat the impact of intermittence of certain renewable energy sources (for instance, solar energy converters, wind energy converter, etc) by doing an equivalence between the actual intermittence and the usual nonendoreversible parameter of finite time thermodynamics.