- THE MAGAZINE
End uses are typically not co-located with energy sources, source energy processing or source energy conversion facilities. Because of these spatial discontinuities, energy transport is almost always a requirement for source utilization. Some forms of energy transport much more easily than others, both in terms of energy required for transport and capitalization for the transport infrastructure.
While electricity is often considered the most easily transportable form of energy, that is not always the case. For a project evaluating future energy solutions for a Middle Eastern country, distributed cogeneration plants co-located with sewage treatment plants and district thermal plants were suggested. Initially, this approach was discounted because natural gas is typically not distributed in urban areas in this region. Local regulations, however, mandated that electric transmission through urban areas be underground. The cost of the underground 300-kV transmission lines from remote power stations was significantly greater than the cost of underground gas pipelines, with the added benefit that co-location of the power generation with district thermal plants, within their limited radius service area, allowed recovered heat from the generation of power to be utilized for generating cooling and for distribution of domestic hot water.
Transport includes moving energy from its initial source to its ultimate end use. Along the way, conversions may be necessary to traverse obstacles and conduits specific to certain segments of that path. To some extent, transport issues are related to the energy density issue (see below in “Energy Storage”). Efficient transport of energy over anything beyond a short distance requires a certain minimum energy density. A good example of this issue is the utilization of low-grade waste heat (less than 150 F) from cogeneration. Unless the cogeneration plant is located immediately adjacent to the thermal consumer, transport energy will be greater than the value of the recycled “waste” heat. Similarly, utilization of agricultural waste for cellulosic ethanol production requires location of the ethanol generator immediately adjacent to the source of agricultural waste. The energy density of the waste is so low that the energy required for transport beyond more than a short distance will be greater than the energy recovered from the waste.
Just as there are spatial discrepancies between energy sources and end uses, there are also temporal discrepancies. These discrepancies are particularly acute for renewable sources that typically provide energy in a randomly varying fashion. A second form of temporal discontinuity involves energy supply of mobile or temporarily connected end uses. These end uses are “refueled” from time to time by connection to a source stream, but their operation schedule is independent of that connection. Examples of this phenomenon range from cell phones to automobiles to houses heated by fuel oil.
The critical parameter with respect to this issue is how much the primary end-use activity schedule can vary from that of the schedule of energy sourcing. In the case of a closed-loop connected system with a random supply source, how radically different can the demand schedule be from the supply schedule—even if the total demand and supply over an arbitrary period of time are equal? For an open-loop or disconnected system, how much end-use activity can occur between “refuelings?” These questions depend on the amount of storage available, which, to a great extent, depends on the energy density of the storage medium measured by cost, volume and weight.
Energy storage technologies have a number of important metrics, including:
- Energy density;
- Round-trip conversion efficiency (ratio of energy extracted from medium to amount of energy required to recharge to same level);
- Lifespan (degradation rate per recharging cycle);
- Speed of charge/discharge
- Storage energy input/output form; and
- Environmental overhead for storage medium.
An evaluation of storage technologies should take into account all of these factors. The above comparison of gasoline to lithium-ion batteries for automobiles shows the characteristics of each technology.
One of our current problems is that our most energy-dense storage media have significant disadvantages from the standpoint of source overhead and conversion efficiency. The most energy-dense storage medium we have is processed nuclear fuel, which requires enormous infrastructure for conversion. It’s very unlikely nuclear-fueled iPods will be available in the near future. Hydrocarbon fuels also have a very high energy density, but issues of carbon release and other forms of pollution into the biosphere make them undesirable.
Perhaps the biggest opportunity for reducing the environmental impact of our energy habit lies with electricity storage. Currently, electrical storage media suffer from relatively low energy density, yet because of the high efficiency of electrical end-use conversion, electrical storage requirements are as little as 25 percent of those for media that require conversion to electricity through heat engines for final end-use conversion. The need for electrical energy storage is made more pressing by increasing percentages of renewable energy in the electrical supply grid. Renewable sources have an unfortunate tendency for rapid changes in their power production. Both a sudden calm or a cloudy weather front moving into an area can have a quick, dramatic impact on production. In order to balance input and output to the grid, backup generation sources must rapidly alter their power output to match the changes from the renewable assets. Often, even the most nimble load-tracking generation technologies, gas turbines, cannot match the change rate of the renewable source, resulting in poor power quality, voltage and frequency variations.
Utility-scale electrical storage, with high charge/discharge rates but with storage capacity of only a few seconds, greatly improves grid stability during these events. Several utilities in Europe and Japan have installed megawatt-hour capacity batteries to help maintain grid stability. The technologies involved, sodium sulfur batteries and flow batteries, do not have remarkable energy density, but that isn’t important because they are stationary. Their important characteristics are overall capacity and charge/discharge rate.
While electricity is the most versatile, efficient and easily transportable energy medium for direct conversion to most end-uses, it doesn’t store very well, requiring conversion to other forms for storage. Electricity has limited application to non-connected (transportation) end use because of low energy density of current storage media. Many energy sources at some point are converted to electricity somewhere in the supply chain from source to end use, either in response to transport requirements or end-use conversion requirements. Most renewable energy sources, with the exception of low-grade solar thermal, are converted first to electricity after harvesting. Because of the multiplicity of non-greenhouse-gas-generating sources that convert initially to electricity, reduction of societal greenhouse gas production is more easily approached through “greening” of grid power sources than it is through “greening” of gas pipeline systems.
Many energy conversion processes involve the production of significant amounts of waste heat that is usually wasted into the atmosphere or into surface water resources. Co-location of electric generation sources with low-grade heat consumers enables cost-effective energy conservation through heat recovery. The current trend for energy conversion location puts our renewable conversion devices (wind turbines and photovoltaic cells) on our buildings, diminishing the devices’ performance while remotely locating our heat engine electric generation, precluding utilization of recovered waste heat. Wind turbines belong on the Great Plains, mountain ridges or the continental shelf. Photovoltaics should be deployed in low-latitude deserts with a south-facing slant. Heat engine electric generation belongs in our buildings in cities, where the waste heat can be harvested to heat our bathwater.
Some widely heralded innovations come up short when exposed to the analysis described previously. For example, while hydrogen has a high energy density by mass, has less than 20 percent the energy density of gasoline by volume, doesn’t store or transport easily, and has a round-trip efficiency (reversible fuel cells) that is worse than lead-acid batteries (50 percent), it isn’t a source because it is produced by applying electricity to water or by conversion of natural gas and it has sourcing efficiency issues.
The popularity of hydrogen as a potential remedy for our energy woes is based upon a simplistic analysis focusing on only one aspect of its entire supply chain: direct conversion of hydrogen to electricity produces only water and no greenhouse gases. Positing it as a potential cure ignores its deficiencies for conversion efficiency, sourcing overhead, transport and storage.
Application of this framework gives significant insight into the nature of our energy “crisis.” Some of the immediate conclusions are:
- Single technology “revolutions” are unlikely to significantly impact the global energy and greenhouse gas equation unless they address conversion efficiency, sourcing overhead, transport and storage.
- Energy analysis of conversion processes uncovers simplistic efficiency fallacies and reveals hidden opportunities for savings.
- A revolution in electrical storage energy density would bring electric conversion efficiency and sourcing overhead advantages to both mobile and stationary end uses, resulting in significant reduction of greenhouse gas generation.
- Optimal siting of renewable and conventional electric generation assets can reduce greenhouse gas production by optimizing efficiency and enabling waste heat recovery.
While recent technical advances for fossil fuel production have likely delayed the dreaded onset of “peak oil,” issues with greenhouse gas production and climate change have been exacerbated. Emerging economies have little interest in energy conservation that precludes “first world” lifestyles. Reconciliation of world needs with legitimate individual desires requires consideration of global energy problems in a systemic context. Potential improvements to our energy problems should be evaluated for their effectiveness not as isolated processes, but as components of a global supply and consumption chain.