Review PaperCarbon dioxide based power generation in renewable energy systems
Introduction
Productivity whether it be agricultural, industrial or commercial is given enormous importance in developing countries as a path to prosperity. Availability of high grade energy, and specifically electrical power is intricately interlinked to productivity. While demand for the former continues to grow at a very rapid rate, environmental issues are inhibiting large scale (at several 100s of MW level) generation of electricity using commonly opted resources such as hydro-, fossil fuel or nuclear. As a result, more governments are turning to renewable energy sources for electricity. Among them, solar energy is the most sought after. The quickest way of electrifying has been through photovoltaics, though, long term sustainability of this option is also to be weighed vis-a-vis use of materials like tellurium, indium and cadmium which are in limited supply and some of them may be associated with health hazards [1]. Comparatively, solar thermal based electricity generation has only a limited penetration. Most of them use concentrated solar energy for generating steam with the rest of the processes being as in a Rankine cycle [2]. Organic Rankine cycle (ORC) power plants powered by waste heat or solar thermal energy have also been explored at a few 10s MW level and there are several in operation too [3]. One of the main concerns in ORC power plants is the direct greenhouse gas emission arising out of some of the working fluids (such as R-245fa) which have a high global warming potential (GWP). Environmental impact of solar technologies are also often assessed because there are several issues that need to be considered to establish their viability [4]. A notable feature of solar thermal electricity is that no exotic materials are required in their hardware.
For servicing settlements which are quite far from conventional electrical grid, distributed localized power generation is quite attractive because it obviates losses occurring from long distance transmission followed by local distribution of electricity. Steam based solar thermal power plants become unviable at 100s of kW or smaller levels. ORC power plants need to be operated with working fluids which are either flammable such as propane or toxic such as ammonia if GWP issue has to be addressed. The other option is to search for alternate cycles, e.g.: Brayton. Air is the most sought after working fluid which needs high temperatures – such as those obtainable from combustion of natural gas. With solar energy as the thermal source where temperature levels are at best 800 °C, a large fraction of the turbine work needs to be allocated to the compressor. It is a common knowledge from thermodynamics that to realize a large energy output, one requires a large enthalpy change of the working fluid which can be either brought about by pressure or temperature changes (or both) with temperature effect being much stronger than the pressure effect. It is apparent that the engineering community is challenged to come up with cycles and working fluids for generating solar thermal electricity incorporating totally environmentally friendly disposition.
On the other hand, looking at the end user of energy, among which is refrigeration and air conditioning industry, natural working fluids are in the lime light with carbon dioxide (CO2) emerging as a good choice from every indicator of environmental performance [5]. One must hasten to add that even this sector has been cautious in promulgating this fluid because of its high operating pressure requirements and also higher work of compression compared to halocarbon refrigerants in vogue. More notably, the inhibition arises from transcritical operation of refrigeration cycle and the lack of trained manpower in this area. It is logical to infer that, if CO2 consumes more power during compression than other working fluids, it should also deliver more power while expanding in the same region. Incidentally, CO2 as a Rankine cycle working fluid has been in the reckoning of nuclear power industry for over a decade [6], [7], [8], [9]. More recently, that option now seems to be applicable even in the renewable energy sector because of the possibility of generating concentrated solar power at temperature in excess of 500 °C [10], [11], [12], [13], [14]. This paper reviews the work that has been in progress in this area and presents an overview of prospects and limitations of this option. How the lacuna of high operating pressures can be overcome by using low pressures and thermal compression is also investigated. Another possibility of lowering the upper cycle pressures and simultaneously elevating condensing temperatures by blending CO2 with flammable hydrocarbons is presented. The latter option takes into account the requirement of suppressing the flammability of hydrocarbons and yet retaining their positive thermodynamic features. While the general focus of this paper is on thermodynamic aspect, the fall outs of some of promising choices from the heat transfer point of view are also highlighted.
Section snippets
CO2 as a standalone working fluid in Brayton cycle
CO2 as a working fluid has a wide domain of operation whether it be for refrigeration or power generation. For refrigeration, it is in vogue as a primary working fluid in sub-critical and transcritical zones. It is also used as a secondary refrigerant in supermarket applications. In view of peculiarities of this refrigerant, Srinivasan et al. [15] proposed the temperature-exergy plane for obtaining a better perspective of the cycle than the conventional pressure-enthalpy plane. In the area of
CO2 as a Rankine cycle fluid
A lot of research has occurred with CO2 as a Rankine cycle working fluid where heat rejection can occur at subcritical temperatures implying ambient temperatures being quite low (well below critical temperature of CO2 ∼31 °C). Desired upper cycle operating pressures can also be <100 bar. But in this Rankine cycle, heat addition is in the supercritical region. Angelino and Invernizzi [33] report an analysis of such a cycle with liquefied natural gas (LNG) as the heat sink. This cycle has the
CO2 as a blend with other working fluids
In all the afore mentioned applications of CO2 as a Brayton cycle working fluid, the negative features are either high operating pressures if used in mechanical compression or large heat inventories if used with thermal compression. Thermal compression also is still conceptual and it is unlikely that in the immediate future any practical power plants will be built. If used as a Rankine cycle fluid, the sink temperatures have to be well below critical point of CO2. In this context, it is
Heat transfer issues
There will be at least three heat exchangers in the CO2 Brayton cycle: (i) the regenerator where the high pressure streams emerging from the compressor is heated by the hot low pressure exhaust stream of the expander, (ii) enthalpy enhancement of the high pressure stream by the concentrated solar heat derived at the receiver and (iii) heat rejection in the gas cooler before CO2 enters the compressor. Since the expansion ratios are quite small, the low pressure stream at expander exit is quite
Concluding remarks
In this paper range of possibilities of using CO2 as a working fluid for solar thermal power generation have been reviewed. Although, in complete supercritical state it has several benefits, inhibiting factors are high operating pressures and need of higher source temperatures. In the transcritical non-condensing mode, with some sacrifice in efficiency more amenable turbine inlet pressures can be realized. Complete sub-critical operation is impractical since the work of expansion is virtually
Acknowledgement
This research is based upon work supported by the Solar Energy Research Institute for India and the U.S. (SERIIUS) funded jointly by the U.S. Department of Energy subcontract DE AC36-08G028308 (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, with support from the Office of International Affairs) and the Government of India subcontract IUSSTF/JCERDC-SERIIUS/2012 dated 22nd Nov. 2012. In addition, the authors would
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