An essay on evaluating renewable energy systems.
Energy Sources and Energy Transport Media
The main sources of energy in use today are solar, nuclear and a very limited amount of tidal energy. All other energy products like fossil fuels, hydrogen and bio-fuels are not energy sources, but energy transport media. In the case of fossil fuels there is a large amount of solar energy concentrated over time and they are relatively easy to harvest, but they are not an energy source. The length of time it takes to form oil, natural gas and coal makes them a "non-renewable" resource and the carbon stored in these underground sources has the potential to change the atmosphere when these fuels are used.
Heat Engine Understanding
A basic understanding of heat engines is required prior to evaluating renewable power generation systems. An analogy that can be used to explain heat engines and thermodynamics well enough to understand renewable power systems is comparing them to a hydroelectric dam:
- In a hydroelectric dam, you can convert a portion of the potential energy of water flowing downhill into work. You can only convert the energy when the water is flowing downhill and you cannot convert all of the energy because that would stop the water from flowing. The maximum efficiency is the head difference (high and low water points). Unless the low point of the dam is at sea level, you are not getting all of the potential energy out of the water. Water flows downhill with a force and it is possible to reverse this, but that requires input energy.
The 2nd Law of Thermodynamics and Carnot Efficiency of heat engines have the same major points:
- You can only convert some of the heat to other work while it is moving from hot to cold and the maximum efficiency is the difference in the high and low temperatures relative to absolute zero.
- Anywhere there is a temperature difference there is a possibility to allow the heat to move from hot to cold and in the process convert some of that heat to do work. The larger the temperature difference and the ease at which the heat can be moved are what define the potential power output of the heat engine.
Electricity as Energy Transport
Electric motors and devices are not heat engines and the conversion from electrical to magnetic to mechanical force is very efficient in both directions compared to heat engines. In fixed locations and rail transport, electricity is a very efficient means to transfer energy. In transportation, the energy density of electrical transport systems (battery weight) is poor compared to hydrogen and carbon fuels and the weight and size of electrical storage systems becomes an important efficiency factor.
Energy Transport Media Heat Engines
In traditional internal and external combustion engines powered by hydrogen and carbon energy products, although the temperature of combustion is relatively high, usually the cold point is the ambient air. Because the efficiency is related to absolute zero (-273C) the efficiency of these engines is limited to the percentages near what we see in current systems. The fact that ambient air is much warmer than absolute zero is the major reason internal combustion engines have 15-30% fuel efficiency. In both non-renewable and renewable fueled systems, the efficiency of the consumer engine is important to the efficiency of the entire system. The energy density of the fuel and the safety of transport are also important.
Renewable Energy Systems and EROEI
In renewable energy systems, the concept of efficiency is different than energy product engines because the input energy is renewable. The "efficiency" of the system is limited to the temperature difference, the amount of energy to transfer the working media, the amount of energy that went into construction of the system and the availability of the construction materials.
The economic evaluation is also different for renewable systems built from common materials where the major cost of these construction materials is energy in extraction, manufacture and transport. The current economy is based on non-renewable energy and this makes it difficult to evaluate non-trivial renewable systems by monetary means. As the economy is shifted towards renewable energy, the energy "cost" changes . A simpler evaluation approach is to measure total energy input of construction and maintenance versus energy output. The term Energy Returned On Energy Invested (EROEI) is used to describe this idea.
These points are fundamental in evaluating a renewable energy system and can be applied to all of the current renewable energy ideas. They can also be used as basic design criteria for a renewable system. We want to find the largest temperature difference available, move as much heat as possible while expending as little energy as possible to move the heat transport media and we want the collection system to capture as much solar energy as possible for the energy required to build the infrastructure involved. If the system is built from common materials and can produce enough energy to construct a like system within a reasonable length of time it is feasible and will be able to "reproduce" itself. Human effort to build systems is a renewable resource. The EROEI of a system is very important, but is a very narrow evaluation of a complex problem and there are many other factors that need to be considered when evaluating energy systems.
Classifying Energy Systems
Energy generation systems can be broken down into two classes and a hybrid of those two classes. Systems that collect the solar energy directly and systems that leverage some existing feature of nature. As an example, Solar Photo-Voltaics and Solar Steam Turbine systems directly convert solar energy into electricity and fossil fuels, wind turbines, hydroelectric dams, ethanol, biodiesel and wave (not tidal) power would be examples of indirect solar collection.
Nuclear energy requires a naturally occurring non-renewable fuel. Uranium has a relatively low EROEI for known supplies, but again that isn't the only consideration.
Fossil fuels are very concentrated solar energy and the EROEI to extract and use these fuels is minimal. This makes it very difficult to design a system that captures solar energy "real-time" that can be competitive with energy concentrated over many thousands of years. The recent trend towards renewable energy is due to a realization that EROEI doesn't account for all of the factors and the true "cost" of using these non-renewable energy products is difficult to calculate. Generally renewable systems that leverage an existing feature of nature like hydroelectric dams and wind turbines have better EROEI than direct solar energy systems.
Factors for Evaluating Renewable Energy Systems
- EROEI Energy Returned On Energy Invested which is the system energy output over construction and operation energy input. This is the term that is usually used in isolation when comparing energy systems, ignoring the rest of the factors.
- Location Independence of the generation system. The construction energy of transport infrastructure and loss of energy during transmission is a major factor in total system feasibility. The closer the renewable power generation system is to the consumer, the more efficient the total system is. If the system is used to manufacture energy transport media, the distance the energy product needs to be transported is also important.
- Scalability and Availability of construction materials and input media. If the required construction materials are rare or require a lot of energy to locate and process, this affects the efficiency of the system. If the system is built from common and recyclable materials the system will scale well. In the case of energy media manufactured from organic sources (like ethanol, bio-diesel and bio-mass), the scalability and availability of these sources is important. If the organic input media is a waste product and it may be converted into a usable energy product without a large environmental impact, the scalability is less important than the use of an otherwise wasted product.
- Reliability: If the system output is intermittent (i.e. only producing power when there is direct sunlight or the wind is blowing) either an energy storage system needs to be incorporated or the system is limited to supplementary power generation. There is a limit on the percentage of intermittent electrical power generation that may be tied to the electrical grid before it becomes unstable. The guideline from the utility companies is at around 10% intermittent generation to maintain grid stability. The other portion of reliability is related to serviceability and generally the less moving parts and simpler the system the less chance that a component or the system will fail.
- Serviceability: If the system is serviceable and individual components can be repaired or replaced the whole system has a better energy efficiency than systems that are not serviceable and need to be replaced completely at the end of there usable life span.
- Environmental Impact: Although most renewable systems have a lower impact on the environment than fossil fuels, structures like hydroelectric dams usually require major disruptive changes to waterways and the local environment. The manufacture of the components may also have an substantial environmental impact and in the case of converting an existing waste product to fuel there may be a positive environment change.
- Aesthetics and architectural design of the system are also very important to society.
- Transportability: The ease at which an energy transport media can be safely transported and stored as well as the energy density of the media.
- Implementation: The amount of effort required to convert traditional fueled systems to the renewable product.
- Efficiency of Consumer Engine: The total system efficiency is affected by the engine used to convert the energy transport media to work by the consumer.
- Complexity of technology and whether the system requires highly specialized equipment to produce and whether this equipment is available to the general public.
- Intellectual Property ownership and other political factors affecting whether the technology can be replicated by the community or will be controlled by agencies that will arbitrarily set the market price once the system is in place.
- Security: Large centralized power generation systems and processing/refineries are more vulnerable to major attacks than interconnected community systems. The Internet is a good model of a distributed system limiting single points of failure and is very difficult to completely disrupt.
Based on the outlined criteria a scorecard can be developed to evaluate renewable energy systems. This scorecard would be relative to the consumer location and use for the energy. Some systems will score differently depending on the location and usage. Some systems will score better as general solutions, but they might not be the best solution for isolated applications.
This essay will not attempt to score individual renewable energy systems. The main point is that evaluating energy systems by Energy Returned Over Energy Invested and ignoring the other factors led us to the extreme use of fossil fuels to supply our energy needs. We are just now learning that this was too simple of an evaluation.
We are not running out of oil next Tuesday. If we are going to start implementing renewable replacements for fossil fuels, it would be good to learn from our mistakes and evaluate these systems on more than EROEI before we start putting them in place.