For supercritical CO2, a small change in temperature or pressure can result in large change in density, especially in the state close to the critical point. The large change in density can easily induce the natural convective flow. In this chapter, a solar water heater using supercritical CO2 which is originally designed and constructed will be introduced. The solar water heater is a closed loop system with main components of an evacuated solar collector and a heat exchanger. The working fluid of CO2 is naturally driven by the large change in density with absorbing and transporting heat in the solar collector. And the heat energy (hot water) is produced by exchanging the transferred heat with water in the heat exchanger. This chapter will describe the typical system operation and performance at different season and climates.
TopIntroduction
In recent years, the global warming, atmosphere pollution and energy depletion have become critical issues in the world. The main factor leading these issues is that our life strongly depends on the consumption of fossil fuels such as coal and crude oil. Larger amount of fossil fuels become consumed with rapid economic development and growth of population. In order to establish the sustainable energy society without the strong-dependency of fossil fuels, it is important to develop environmentally friendly renewable energy systems which exhaust no greenhouse effect gas. Solar energy, wind energy, biomass and so on have attracted much attention as renewable energy. Among them, the amount of solar energy is rather large comparing other renewable energies. The amount of solar energy striking the Earth in one hour is 4.3×1020 J/h, which value exceeds all energy consumed on the planet in one year of 4.1×1020 J/year. Therefore, the solar energy is the one of promising environmentally friendly and sustainable energy, and a lot of research and development of the renewable energy system using solar energy have been promoted.
There are different methods for utilizing the solar resource such as solar electricity, solar thermal energy, and solar-derived fuel from biomass. Among these solar resources, solar thermal energy can be efficiently recovered into other energies (Zhang et al., 2005; Zhang et al., 2006; Zhang et al., 2007; Zhang et al., 2008). Solar thermal utilization system can be categorized into low-temperature solar thermal systems, which may not involve sunlight concentration, and high-temperature solar thermal systems, which generally require concentration of sunlight. Low-temperature solar thermal systems have the potential to supply a significant number of households and commercial buildings with heating, cooling, and refrigeration. Solar water heater is one of the most promising low-temperature solar thermal systems (Davidson et al., 2002; Chen et al., 2003; Greffet et al., 2002; Mancini et al., 2003; Kalogirou, 2003). During recent years, a lot of studies were carried out in the field of solar water heater (Budihardjeo & Morrison, 2009; Dubey & Tiwari, 2008; Anderson & Morrison, 2007; Esen & Esen, 2005; Morrison et al., 2005; Morrison et al, 2004; Dahl & Davidson, 1997; Moson & Davidson, 1995; Rosengarten et al.), which are mainly for the solar water heaters of water-in-glass evacuated tube type.