Combined energy techniques

Side effects turned into advantages

History is full of stories of men feverishly searching for the magic trick that will solve all of our energy problems. Those quests have generally been whistled back by the laws of physics. These laws are what they are and we have to cope with them; our energy future will not be built by revolutionary solutions plucked out of the blue.

Perhaps the most intelligent solutions are not those that try to breech the limits of nature, but those that make maximal use of what nature has on offer. These are the stories of side effects turned into advantages, resulting in efficient combined techniques.

Systems with heat recuperation

Perfect examples of such combined techniques are those that recuperate heat that would otherwise be lost. A classic concept is that of combined heat and power production. The laws of physics and thermodynamics state that heat can never be turned into electrical energy with 100% efficiency, since the latter has a higher exergy level. Some heat will always be lost. However, this heat can be effectively used, or at least some of it. That is what happens in Combined Heat and Power (CHP) plants, which are supplying heat for industrial purposes or for city heating.

Heat can also be recuperated from cooling applications. In many cases, heating and cooling are required on the same location, for instance in the food processing industry. Such an installation will have a much higher combined efficiency than stand-alone cooling or heating systems.

Using the sun to compensate for its discomforts

Another category of combined techniques are those that couple the disadvantages of the sun with its advantages. In regions where the sun shines high and hot, people try to create reasonable living conditions by developing water supply systems and air-conditioned buildings. Both of these methods require enormous amounts of energy. However, the source of the trouble — the sun — can also provide a simultaneous solution: the sun’s energy can be captured to produce useful heat and electricity.

This occurs, for instance, in solar air-conditioning systems. Since the output of photovoltaic (PV) panels will be at its highest when air-conditioning is the most needed, they make a perfect match. They can be connected to a small micro-grid with a weak coupling to the national grid to compensate for any shortages or excesses in solar electricity production.

A similar example is the solar powered water pump. A greater amount of sunshine produces a greater need for water for irrigating agricultural production. Providing the energy for water pumps with photovoltaic panels is consequently a perfect solution.

Solar desalination is yet another excellent example of a combined technique. Not only do places with a shortage of potable water often have abundant sunshine, solar desalination can also combine electricity and heat driven systems. A thermal Concentrated Solar Power (CSP) plant initially produces heat from the sun’s irradiation, and then uses this heat to produce electricity, a step that entails heat loss. Consequently, the overall efficiency will increase when both heat and electricity are used. This is the case in a desalination plant that makes use of two different techniques: electricity is used to drive the compressors of a reverse osmosis plant, and the heat is used in a traditional multi-effects desalination plant.

Carbon storage and oil extraction

A very different type of combined technique is Carbon Capture and Storage (CCS) and Enhanced Oil Recovery (EOR). Injecting CO2 into oil wells to increase production is a technique that has been widely used since the 1970s. With today’s struggle to reduce carbon emissions, it feels a bit awkward to produce new CO2 for drilling oil. Hence the idea of creating a complete cycle: capture the CO2 at the end of the stack when burning the oil and then inject it back into the ground to enhance the production of new crude oil.

Despite the apparent value of the basic premise, this concept has been criticized on a number of grounds. The first reason is doubt about the cost and feasibility of carbon capture itself. A second criticism involves the safety of storing CO2 in underground wells: what if such a well leaks? The last and most fundamental critique is that capturing CO2 will most probably only be economically viable in coal-fired power plants, not in applications burning oil. When CO2 is injected into oil wells, it increases the CO2 content of the harvested oil, and only a small percentage of the CO2 stays underground. Consequently, if the circle is not made complete, the CO2 that is released when burning this oil is not recaptured and the efficiency of the whole operation in reducing the CO2 content in the atmosphere will be very low.

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