21st-Century, Stable, Thermal Energy Storage

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The “dawn of new idea” may be a cliché that is appropriate to recent developments in photo chemical energy storage. Researchers at MIT have been working on ways to store the sun’s energy in chemical bonds. These developments have been characterized as a “heat battery” or a “solar fuel.” This development could very likely see another MIT spin-off in the energy storage business (like A123 systems).

The claim of a “game-changing” or “disruptive” technology should not be made lightly. Also, we should be reasonable in our expectations. Cell phones, personal computing and the Internet were certainly game-changing technologies, but they arrived on the scene in a virtual vacuum. As we begin to wake up to the advantages of energy storage, we find that the scene is also crowded with competing technologies. Heat energy storage is also as old as the Earth. The uniqueness of MIT’s research becomes clear with a little background.

Energy & Energy Storage

Some forms of energy have an associated storage capacity. The mechanical energy in a clock’s movement can be stored in a spring or the weight of a pendulum. Other forms of energy are more complex. Electrical energy we can store as a charge on a capacitor or we can convert it to chemical energy that will produce electricity in a storage battery. When transferring energy from one form to another, there are typically also losses of efficiency.

Heat is a form of energy that we use a great deal. In some northern climates, only about 15% of residential energy is for electricity while more than 65% is used for heat and hot water. Indirectly, we use heat to drive machines and produce electricity. The sun’s energy comes to us in the form of heat and light. With PV panels, we turn the light into electricity. With concentrated solar power plants, we much more efficiently turn the sun’s heat into electricity.

Sensible Heat Storage

There are a number of ways that heat is stored. The most common is “sensible heat storage.” This is the heat that is stored in substance as it gets warmer. Materials differ in the amount of heat they can contain. This is measured as the “specific heat value” and water is about the best material we know for storing sensible heat. It is for this reason that heat storage tanks in residential solar thermal systems contain water. This is also why “thermal mass” in passive solar homes is made from large tubes of water. Stone and masonry are the next common materials at about half the specific heat value of water. Homes made of these materials tend to heat up and cool slowly as the thermal mass buffers the more rapid heat changes in the building air.

We have done some remarkable things with sensible heat storage. With a large enough mass and enough insulation we can even store the summer’s energy to heat homes in the winter. But to be economically practical these are often community-wide systems. Thermal mass is not stable over the long-term. This is why we also use insulation to restrict heat loss.

Latent Heat Storage

“Latent heat storage” may seem like a bit of a magic. The energy needed to change the state of a substance is greater than its sensible heat value. Water must give up roughly 1 calorie of heat for each cubic centimeter (about 1 gram) that falls 1 degree centigrade, until it changes state and turns to ice. The change in state gives up about 80 calories. At this time 1 gram of water will soak up as much as 80 grams of water at a warmer temperature. This latent heat storage is used in steam heating (and ice cooling) systems. Materials are designed with their physical change in state to correspond to a temperature we want to maintain. Waxes are used for room temperatures and salts are used at higher temperatures for Concentrated Solar Power (CSP) storage. These are called “Phase Change Materials” (PCM). We will increasingly see PCM incorporated into our clothing and our environment.

Air conditioners use the principal of latent heat storage to move heat from the evaporator to the condenser through a compressor that changes the refrigerant from a gas to a fluid. A heat pump can reverse the cycle and change which side is to get warm and which side is cool. There are also “absorption” air conditioner systems that convert heat energy directly into cooling in our environments.

Chemical Heat Storage

Chemical heat storage takes advantage of those chemical reactions that give off heat or soak up heat. Plaster of Paris, when setting, gives off heat. At the local pharmacy you might buy a pack that you shake (to mix chemicals) and it becomes either hot or cold. Other forms of heat storage use insulation to prevent energy loss into the environment. This is not necessary with chemical heat storage.

Burning fossil fuels is also a chemical reaction that gives off heat. But three drawbacks of our most common fuels are that they use elements not part of the initial compound (oxygen from the air,) give off waste materials (emissions and pollutants), and they are not an economically reversible reaction. This is essentially what makes fossil fuels not sustainable.

There are reactions that are reversible and sustainable without using air or producing pollutants. Other forms of heat storage don’t actually remove heat from an environment but chemical storage changes heat into another form for easy long-term storage. Some of these systems are described as “chemical heat pumps,” and are advocated among other forms of heat storage as ways to store energy for our electrical grid.

Recent MIT Research & Photo Chemical Heat Storage

About a year ago, MIT associate professor Jeffrey Grossman with associates came to a new understanding of how light energy can be stored chemically. The recent use of carbon nanotubes and an association with postdoc Alexie Kolpak has lead to the design of new methods and chemicals for the process. Photo chemical heat storage is a simpler process than first capturing heat and then using another mechanism to transmit it and another to store it.

We can imagine a new type of solar panel that captures and then stores heat from the sun. These would share modular and scalable advantages with PV panels. Unlike PV panels, we would now also have a product that (like coal, oil or gas) holds energy in chemical bonds. Unlike fossil fuels, once the heat is released they can be recharged. The process is reasonably compared to a light rechargeable heat battery or a light rechargeable fuel.

One of the problems we have with solar energy is that it has the most potential in many remote, dry areas. Power must be concentrated and transmitted to population centers. Expensive and less-efficient energy storage is not a preferred option though a necessary component. Photo chemical storage has potential for all of these issues. Passive solar homes can be self-sufficient in heating and cooling. But in crowded urban environments space and solar access are not always an option. Photo chemical storage can address these problems. Solar thermal panels are sized for about 80% of demand. But this risks overheating them in the summer and having insufficient capacity in the winter. Such systems typically use a hot water tank as a thermal mass. A system which replaced the hot water tank with chemical batteries could be shut down while you were away but give heat and hot water when restarted. Such a system could even store summer heat for use in the winter.

In transportation, our present internal combustion engines convert chemically produced heat to motion. We have other engines that can do a similar job more efficiently. Photo chemical heat storage could be used to drive mechanical motion through a Stirling cycle quasi turbine rotary engine with no pollutants. Having an established method of changing heat to motion, it is only one more step to connect a generator and change the heat into electricity. There are also other recent methods to convert heat into electricity.

As with electrical energy storage, much of this depends upon future advancements in energy density. Even at this early stage, energy density is comparable with lithium-ion batteries. The future of this technology has much more to offer.

Source: Research update: New way to store sun’s heat

Additional reading:

  1. Storing Solar Energy Indefinitely: A “New” Energy Storage Approach from MIT Moves Forward [VIDEO]
  2. MIT Energy Storage Discovery Could Lead to ‘Unlimited’ Solar Power
  3. Another Cheap Way to Store Solar and Wind Power
Photo via Flickr user mary jane watson

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