Sustainable energy: of course; but how?The meteoric and continuing rise in carbon-based energy utilization worldwide and the resulting global climate change implications constitute the defining problem facing humankind today. These demand scenarios and the emissions from carbon-based energy utilization have been documented in depth, with implications ranging from changes in lifestyle to cataclysms. Instead of further restatements of the problem, strategies are needed to address the opportunities for innovation presented by this scenario in energy conversion and utilization. The challenge in energy is not just one of more and more generation from fast-depleting nonrenewable sources or burgeoning renewable sources. A more immediate and practical opportunity is the end-use aspect. Energy utilized in the thermal form directly accounts for at least 87% (coal 24%, oil 36%, natural gas 21%, nuclear 6%) of the world's current (2003) and projected (2030) primary energy supply (WEC 2006). Many renewables also traverse the thermal pathway, increasing the thermal fraction further. While this preponderance of thermal energy comes from relatively few sources, there are infinite routes for energy utilization and conversion, offering ample opportunities for innovation, especially in the HVAC&R field. What emerges from these considerations is the need for a focus on sustainable thermal energy transformations. The case of electricity generation offers some key insights. The net end-use electricity for residential, industrial, commercial, and transportation sectors in the United States in 2005 was 13 quads, whereas the corresponding conversion losses (primarily thermal-to-mechanical, and governed to some extent by thermodynamics) were 27 quads (EIA 2006), representing an aggregate conversion efficiency of ~1/3. Recovery of ~2/3 of this source energy down to much lower temperatures than currently utilized presents significant opportunities for innovations in energy recovery.
Current energy generation (power plants) as well as utilization (e.g., household water heaters) patterns directly lead to considerable wasted energy either at medium (electricity generation) or high (water heater) availabilities. Large-scale centralized power plants and distant, disperse end-use locations also constitute an inherent mismatch between thermal needs and waste heat streams. But these are not limitations imposed by the laws of thermodynamics. By designing thermally cascaded end uses, energy losses can be reduced to the absolute minimum, thus utilizing the entire availability of the source. Simultaneous deployment of a distributed generation infrastructure would match generation to end use. By developing technologies for end-use applications aggregated across the temperature spectrum around these distributed power generation sources, high source efficiencies can be achieved. Thus, the really high temperature source would be used for electrical energy generation; followed by intermediate temperature utilization for the generation or process steam or district space-heating or cooling loads in residential or commercial communities; followed by low-temperature utilization for hot water supply, drying, desiccant regeneration, and a variety of other uses. Such efforts would lead to sustainable urban infrastructure with small energy footprints through communities planned around distributed generation and consolidated utilization. Such a matching would extract the last useful Joule from the energy source. This near-lossless energy use approach offers opportunities to reduce energy and materials usage at each stage including generation, conversion, transmission, storage, and utilization.
Almost all renewable, carbon-neutral, and environmentally benign energy sources are intermittent and diffuse, while energy is utilized at high densities. The lack of predictable, reliable, and continuous availability of renewable energy sources presents significant hurdles to implementation in practical systems. Development of on-demand, load-following energy by converting renewable energy sources to portable, dense forms of energy is a key challenge.
The compression in the temperature range of individual processes necessitated by the cascaded overall thermal energy utilization concept described above requires that a) transfer processes should be intensified to increase heat and mass transfer coefficients; b) surface areas, and thereby surface-to-volume ratios, of devices must be increased to achieve the transfer rates in viable packages; and c) the available temperature differences must be utilized optimally.
To address advances in energy utilization, research on thermal and thermochemical processes is required. On a broader scope, this may be viewed as engineering carbon sources and sinks toward carbon closure, where carbon is simply a carrier of energy in a fully recirculatory mode. To this end, renewed emphasis will be placed on low-grade waste heat recovery, energy harvesting, and amplification and boosting of the availability of such energy sources. These include the plentiful opportunities in industrial (e.g., materials processing and fabrication), commercial (building energy systems, food processing, storage and transport, data centers) and residential energy recovery. Vehicular emissions reduction through thermal storage, energy recycling, and efficiency improvement represent other avenues. With the lower temperature differences across which the lossless thermal energy cascade systems with aggregated end uses must function, processes with high heat transfer coefficients such as phase change at the microscales assume considerable importance and may serve as enablers for the feasibility of high-flux thermal systems. Thermal process intensification techniques at the microscales, when implemented in larger systems, will magnify the advantages of microscale heat transfer several-fold, improving system specific power and energy densities and even system reliabilities.
In the arena of energy-intensive devices and systems, thermal/thermochemical energy storage systems are sorely needed. Research should include reduction of losses during charge, discharge, and dormant storage through fundamental advances in material properties. Large surface-to-volume ratios offered at increasingly small scales should be exploited to increase bulk capacities of storage devices. Successful development of these devices will eventually see application in the spatial and temporal concentration of renewable energy as well as in the harvesting of low-grade heat for subsequent utilization to yield superior overall source utilization efficiencies.
The need for efficient utilization of thermal energy also engenders renewed interest in thermally activated cooling and heating systems using absorption, adsorption, and other thermodynamic cycles. The improved heat and mass transfer made possible by microchannel and microscale phase change enhances the economic viability of such systems. Microchannel-based systems also usually have the advantage of reducing fluid inventories, material utilization, and environmental impact. In addition, heating and cooling systems using natural refrigerants and other novel working fluids and cycles as well as integrated water-heating and space-conditioning systems, again facilitated by improved heat and mass transfer devices, will reduce the ecological footprint of residential and commercial building energy systems. Other means to reduce energy consumption in space conditioning include wearable power and comfort cooling systems; micro-cooling environments; and combined cooling, heating, and power systems, especially in multi-use commercial facilities such as hospitals, industrial parks, and campuses. Other sustainable thermal systems with major energy impact on the worldwide urban infrastructure include green roofs facilitated by the understanding of transpiration cooling and improved energy harvesting in buildings for net zero energy use. Advances in microscale pumping and compression will also supply critical needs in developing countries such as portable medicine delivery and storage.
Many of the processes, devices, and systems outlined above require the development of innovative heat transfer fluids, colloids, surfactants and additives, natural refrigerants, and other enabling materials. Structured surfaces and coatings for catalysis, energy storage, process intensification, membranes for osmotic processes, desiccants for species and heat transport, and metal hydrides for storage are other examples. On the other end of the spectrum, the ubiquitous but grossly underutilized thermal capacity offered by the ground as a cost-effective, reliable, and predictable storage medium must be exploited. Similarly, for low temperature and low energy intensity applications, inexpensive, recyclable, and readily available materials such as paper, plastic, and others should be used as materials of construction for affordable heat recovery devices to enable market penetration to the masses in the developing world.
While the dependence of economic growth on energy consumption is often taken as an absolute truth, it is in fact possible to achieve the same level of satisfaction in the human enterprise without energy gluttony. One only needs to review the relationship between the Human Development Index (HDI) and energy intensity. As shown by Benka (2002), prosperity, human development, and all other monikers used to describe advancement show plateaus when plotted as a function of energy consumption, with countries such as Spain and Italy achieving the same HDI as the United States with about one fourth the energy intensity of the U S. Regional, geographical, climatic, and other factors account for some of the excess energy consumption in the US; however, by no means can these factors account for it all. Scientific breakthroughs, technological innovations, and implementation of existing but underutilized technology must all contribute to the reduction of this energy intensity. In addition, common themes between different sectors, e.g., transportation and building energy, should be exploited to maximize the impact of advanced devices developed initially for one of these applications. Also, the dwindling availability of energy sources moves the market to a more tolerant stance on capital costs (implying the creation of additional markets for advanced components) so that life-cycle costs can be minimized. The role of scientifically sound federal and local policies, incentives, and fostering of entrepreneurship in achieving this goal cannot be overemphasized--in fact, often, the bottleneck is not the lack of technology but instead the lack of availability of, or the misapplication of, mechanisms to facilitate implementation. With the limited budget of energy and materials in the world, the directions outlined above will enable us to wisely control and harness their transformations, challenge the established limiting wisdom, and bring about changes that affect lifestyles but not the quality of life.
Benka, S.G. 2002. Special issue: The energy challenge. Physics Today 55(4):38-39.
EIA. 2006. Annual Energy Review 2005, p. 435. Washington, DC: Energy Information Agency.
WEC. 2006. World Energy in 2006. London, UK: World Energy Council.
Srinivas Garimella, PhD
Srinivas Garimella is a professor and director of the Sustainable Thermal Systems Laboratory, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA.