Solar and wind exergy potentials for Mars

Highlights

• We analyse the exergy of solar radiation under Martian environment

• Real data from in-situ instruments is used to determine the maximum efficiency of radiation

• Wind energy is analysed as a source of energy

• We provide results of the exergy efficiency for a single location and for the whole planet

Abstract

The energy requirements of the planetary exploration spacecrafts constrain the lifetime of the missions, their mobility and capabilities, and the number of instruments onboard. They are limiting factors in planetary exploration. Several missions to the surface of Mars have proven the feasibility and success of solar panels as energy source. The analysis of the exergy efficiency of the solar radiation has been carried out successfully on Earth, however, to date, there is not an extensive research regarding the thermodynamic exergy efficiency of in-situ renewable energy sources on Mars. In this paper, we analyse the obtainable energy (exergy) from solar radiation under Martian conditions. For this analysis we have used the surface environmental variables on Mars measured in-situ by the Rover Environmental Monitoring Station onboard the Curiosity rover and from satellite by the Thermal Emission Spectrometer instrument onboard the Mars Global Surveyor satellite mission. We evaluate the exergy efficiency from solar radiation on a global spatial scale using orbital data for a Martian year; and in a one single location in Mars (the Gale crater) but with an appreciable temporal resolution (1 h). Also, we analyse the wind energy as an alternative source of energy for Mars exploration and compare the results with those obtained on Earth. We study the viability of solar and wind energy station for the future exploration of Mars, showing that a small square solar cell of 0.30 m length could maintain a meteorological station on Mars. We conclude that the low density of the atmosphere of Mars is responsible of the low thermal exergy efficiency of solar panels. It also makes the use of wind energy uneffective. Finally, we provide insights for the development of new solar cells on Mars.

Introduction

Solar radiation has been a source of energy in space missions as a power source for small satellites, for large structures such as the International Space Station, and for Solar System exploration. It has been very useful in Mars exploration, in particular during the Viking mission and in several Mars orbiters and rovers afterwards. The Mars Exploration Rover Opportunity was able to exceed the baseline mission duration from the 90 sols scheduled initially to more than 3500 sols, continuing nowadays.

The complexity of the rovers, and the energy demands of the experiments onboard have increased in the last decades. An example is the Curiosity rover in the NASA's MSL (Mars Science Laboratory) mission [1] currently operating on Mars. As the solar radiation intensity decreases with the square of the distance to the sun, solar energy might become inappropriate to maintain a complex spacecraft. For these reasons, the rover Curiosity is powered by a Radioisotope Thermoelectric Generator (nuclear power) and it is likely that the next rovers exploring Mars will use the same kind of energy source. Although nuclear power could be a partial and temporary solution [2], human colonization of Mars will require a perdurable and renewable source of energy. The transport of nuclear material from Earth to Mars implies large risk and costs. The existence of fossil energy, such as carbon or oil on Earth, seems unlikely to be found on Mars and its transport in spacecrafts is not feasible. Finally, geothermal energy is not feasible on Mars, since no significant geological activity has been recorded on the planet.

The objective of this paper is to investigate the efficiency of solar energy on Mars and in order to do so, it becomes necessary to analyze not only the radiation reaching Mars but also its environment.

In the last decades, the mechanical engineer community put much emphasis in providing a framework to analyze the thermodynamic processes properly. Classically, the analysis of a process using the first law of thermodynamics has been applied to thermodynamic problems, appealing to energy conservation rules. However, the second law of thermodynamics is not applied under that approach and the description of the processes can be improved by its inclusion, carrying out what has been called “second law analysis”. The second law introduces the concept of entropy and deals with the heat lost in a process, i.e., is related to the environment and it is of importance in the quality of the radiation. These second law analysis are developed to minimize the heat lost and maximize the obtainable work. The maximum obtainable work –or availability– is described by the concept of exergy (from the Greek exo – εξo – and energia–ενεργια–); the exergy of a thermodynamic system is a measure of the potential work of the system [3]. For a detailed historic description of the exergy concept, refer to [Rezac and Metghalchi, 2004] [4].

The exergy concept has been applied mainly in engineering thermodynamics, and resulted in a more effective method to analyze heat transfer than energy analysis [5]. In particular, the idea of exergy was also investigated in relation to solar radiation, proving to be a very successful area of research with theoretical and engineer applications. The early ideas of Petela [6] and Spanner [7] in 1964 started an ongoing research on the exergy of radiation, providing methods to evaluate the maximum conversion efficiency of solar radiation with different approaches, including direct and diffuse radiation, blackbody approximations, dilute radiation or semi transparent medium [8], [9], [10], [11]. Although Planck derived originally the expression for the radiation intensity for a monochromatic radiation beam at thermodynamic equilibrium, it has been demonstrated to hold for non blackbody radiation at a non equilibrium condition as well [12], [13], [14], [15].

The exergetic analysis combines the two laws of thermodynamics to analyse the energy exchange in a particular environment, providing a powerful tool to investigate the performance of a device in a system. It has been applied extensively on Earth, applied in studies in Europa, US, India and Turkey for example [5], [16], [17], [18], [19]. The exergy concept has been applied to improve the analysis of thermal processes in many situations considering the environment [20], as for example in solar collectors (Flat-plate, Hybrid PV/T systems or Parabolic) [21] under different meteorological conditions [22]. Besides simple implementations of solar collectors, more complex and more efficient alternatives have been studied for heating/cooling applications [23], which provide a more efficient use of solar radiation.

In this paper, we will focus our study to flat-plate collectors in Mars. Even though parabolic collectors for example could increase the obtained energy on the planet, it has never been tested on Mars due to the technological difficulty of transporting the panels. In the case of Hybrid PV/T systems, which convert solar energy to electric and thermal energy, they require a fluid to operate and have not been implemented on Mars yet either.

To accomplish our goal to analyze the exergy of solar radiation on Mars, it is necessary to know the radiation field environment that reaches the planet. In Section 2 we determine the solar radiation reaching Mars TOA (Top of the Atmosphere) and surface, necessary to determine the obtainable energy from solar radiation, considering the current composition of the atmosphere and modelling the Martian orbital position to determine the radiation at different seasons and locations on the planet. In our analysis, we have used typical values for solar panels to provide accurate values of the obtainable energy by solar stations on Mars.

To continue the analysis, the environmental properties of Mars must be considered in order to provide a comprehensive analysis of the solar exergy on the planet. Even though exergetic analysis is a universal tool based on the laws of thermodynamics, the solar exergy on Mars has not yet been studied in depth [2]. In this paper, we evaluate the spatial and temporal evolution of the exergy on Mars based in satellite and in-situ rover data. The eccentricity of Mars is 0.09331 and the maximum distance to the sun (aphelion) is 1.665861 AU. As the distance increases, the intensity of the solar flux reaching the surface decreases, reducing the capabilities of the solar powered spacecrafts. Hence, the exergy of radiation will be different at different seasons during a Martian orbit. Not only that, the exergy of radiation will change with location; and for a given location, on a daily basis. Using the environmental data provided by the REMS (Rover Environmental Monitoring Station) instrument [24] onboard the Curiosity rover [1] we calculate the exergy efficiency of solar radiation on a single location at different seasons in a daily basis. The knowledge of the temperature, pressure and density of the atmosphere is used along with the radiation reaching the surface of the planet to determine the exergy efficiency at different hours.

The Curiosity rover landed at crater Gale (4.49°S, 137.42°E) on Mars. Although Curiosity data are undoubtedly valuable, they are representative of a single location on the planet. In order to analyse the maximum exergy efficiency of radiation at different latitudes on the planet, we use the data provided by the TES (Thermal Emission Spectrometer) instrument onboard the MGS (Mars Global Surveyor) spacecraft [25], [26] to determine the maximum temperature of the environment through a complete Martian year. In Section 3 we explain the formalism used in this work to determine the exergy of radiation and show the results of the exergy efficiency of solar radiation in Mars.

Another energetic alternative for Mars exploration and future human colonization of the planet could be wind energy. Wind stations are an excellent alternative on Earth and their use on other planets could be a potential source of renewable energy. We have considered that alternative and we show the results of the wind energy analysis on Section 4.

In Section 5 we compare the results of solar exergy efficiency between the Earth and Mars, providing insights for future work and developments that could help to increase the efficiency of renewable energy sources on Mars. Finally in Section 6 we summarize the main results of this investigation.