The majority of thermal energy is discarded
In our laboratory, we mainly carry out research into the effective use of thermal energy. Thermal energy is energy which flows from a place where the temperature is high to one where it is lower. Put simply, this means that the higher the temperature of that object, the higher the quality of the thermal energy.
This thermal energy does not remain in a constant state, but is always flowing from places with higher temperatures to those with lower temperatures.
It is absolutely impossible to stop this flow; but enabling people to make good use of it by controlling the flow, speeding it up or slowing it down, is the most fundamental element of our research.
Put this way, it may sound difficult; but we are already employing various such devices in our everyday lives.
For example, if the air temperature in winter is 10℃, while our body temperature is 36℃, thermal energy flows steadily from the body’s surface into the surrounding air. So we put on a sweater or a down jacket to control the flow of heat.
If we take such adaptations further, we can develop them into research about energy use which bridges gaps in time or geography, including the use of winter’s low temperatures as the thermal source for cooling in the hot summer, or the use of the heat from warm regions as the thermal source for heating in cold regions.
The problem of the effective use of energy resources is part of the context of such research.
For example, the heat efficiency of automobile engines is only around 40% even with the latest technology. If we ask, then, what happens to more than half of the energy converted from the heat generated by fuel combustion within the engine into the work of turning the wheels to make the car move, the answer is that most of it dissipates into the atmosphere.
In other words, more than half of the cost of fuel which we drivers purchase at a gas station turns into what is called “waste heat”: it is just thrown away into the atmosphere in the form of heat.
If we turn to consider the electricity which is delivered to our homes, technological innovation in thermal power stations has allowed heat efficiency there to reach around 60%, but almost half of the energy either dissipates, or flows into the facility’s coolant and is then discarded into the sea or the atmosphere.
The question, then, is whether there is a good way to collect and re-use this waste heat.
Another issue is the reduction of CO2 emissions.
Of course, raising heat efficiency leads to a reduction in CO2 emissions. In other words, raising the heat efficiency of the machines and tools in our immediate surroundings and of facilities such as power stations is an extremely important issue from the perspective of environmental problems, too.
Recent years have seen the advance of social initiatives such as using the heat energy emitted by waste incineration facilities to heat swimming pools or to heat roads in snowy areas.
Research into raising the efficiency of the appliances which make our lives comfortable, such as air conditioners, refrigerators, and freezing equipment, and into using their thermal energy efficiently is also important.
This is because, at the same time as leading to the effective use of heat resources and a reduction in CO2 emissions, these initiatives are also directly related to making our lives more comfortable.
Such research and development aimed at making our surroundings and everyday lives more comfortable is at the heart of what we do in our laboratory.
“Magnetic heat pumps,” aiming to become next-generation freezing and air conditioning technology
One way of using heat resources effectively and reducing CO2 emissions is to raise the efficiency of appliances.
For example, heat pumps are a technology which is used without fail in air conditioners, refrigerators, freezing equipment, and the like in recent years.
The history of the development of this technology is a long one, and it came to be seen as complete. However, we have since learned that the chlorofluorocarbon gases used as refrigerants have extremely potent greenhouse effects, and so finding replacement substances has become an urgent issue.
How does an air conditioner cool the interior of a room? The key point is to boil the refrigerant, which is in a liquid state, sufficiently at a temperature of around 0℃, which is below the temperature of the room.
The air which is taken in by the indoor unit of the air conditioner passes heat to this liquid-state refrigerant by means of the heat exchanger. The refrigerant then boils steadily. At this time, the thermal energy of the air taken from the room is used to make the refrigerant boil. For this reason, the temperature of the air drops, and this air is blown back into the room.
On the other hand, the refrigerant, which has become a gas after boiling with the thermal energy from the air, is sent to the outdoor unit, where the heat dissipates into the atmosphere. The extremely hot gust emitted by the outdoor unit of the air conditioner is the thermal energy from inside the room, which has been drawn out of the air by the refrigerant as though it were a pump.
After dissipating its heat, the refrigerant, which was in a gaseous state, returns to a liquid state in the same way that condensation forms on a drinking cup. It is then returned to the indoor unit, where it boils and becomes a gas again in a cyclical process.
At first, when it became known that the chlorofluorocarbon substance used as a refrigerant had potent greenhouse and ozone depletion effects, substitute chlorofluorocarbons were developed. However, these substitute chlorofluorocarbons also have a greenhouse effect. In developed countries, led by Europe, where regulations are particularly strict, and including Japan, regulations to abolish all these substitute chlorofluorocarbons in principle are coming into effect from 2020. For this reason, research to develop new refrigerants and refrigeration technologies to replace these chlorofluorocarbon refrigerants is being carried out around the world.
“Magnetic heat pumps” are what we are working to develop in response. This is a technology which makes use of materials whose temperature rises when they approach a magnet and falls when they move away from the magnet.
This technology has the merit of having a minimal effect on the environment since it does not use any gases with a greenhouse effect, including chlorofluorocarbon refrigerants. It is also able to produce the same effect as existing refrigeration systems using less electricity. In other words, it saves energy.
The issues with it are cost and, when used for air conditioning in cars, the weight and size of the equipment. We are therefore carrying out joint research together with the private sector, aiming to implement this technology in practice.
Thinking about various ways to use thermal energy through thermal storage technology
Thermal storage technology is another way to use thermal energy effectively. We can think of various methods of doing this, depending on the environment, situation, aim, and so on.
For example, peak electricity consumption is generally between about 2 pm and 3 pm, and power generation facilities are set up to match this. However, the volume of operations falls at night, so a large gap arises between daytime and nighttime.
If, for example, we could make a large volume of ice using the night’s electricity during summer and use it to assist the operation of air conditioning during the day, the daytime consumption of electricity would fall, leading to labor saving for the power generation facility.
Moreover, if we want to use the heat emitted by the sun during the daytime as the heat for our baths at night, we could consider storing heat in water at around 60℃. Alternatively, since the waste heat from steelworks and the like is around 200℃, and so it cannot be stored in water, we could consider storing it in metal.
Selecting the most appropriate storage media in this way, according to the temperature range of the heat which needs to be stored, leads to good heat storage.
In our laboratory, we are carrying out research into turning ice slurry – ice in a sherbet-like state including ice particles – into one such medium. This is because ice slurry has the merits of possessing greater cold thermal energy storage effects than water, and of being more convenient to transport than solid ice.
For example, a large quantity of ice slurry is made in a hospital during the night. If plumbing for the ice slurry is installed in the hospital, it can be used rapidly in the operating rooms and hospital rooms where it is needed. In addition, when cooling affected body parts of patients being transported by ambulance, ice slurry is easier to handle than ice, and is able to cool the area where it is needed more rapidly than water.
The point is that what we want to use is stored thermal energy, and we should think about what medium would be best suited for this.
For example, throwing away hot water after taking a bath is a waste, but we tend to think that reheating it for use the next day would be unhygienic. However, it would be fine if we could use the thermal energy from the previous day’s hot water. If we think of a method of collecting and reusing this, and of a medium for this purpose, it will become a heat storage technology in our everyday surroundings.
In this sense, too, I believe that acquiring knowledge about thermal energy and about the mechanisms such as heat pumps employed in the electrical appliances in our daily lives is also useful for the general public. This is because such knowledge will allow us to make good use of the natural phenomena around us and to make a more comfortable life a reality.
As I mentioned at the start, for example, thermal energy does not remain in the same place. It is always flowing from places with higher temperatures to places with lower temperatures, just like a wind that blows. So if we place a “windmill” which is turned not by wind but by heat in its path, we may be able to generate work or electricity even though it seems at first glance that there is nothing there. I call this a “heat windmill”.
Thinking about ideas like this based on the structure of thermal energy is fun, and I believe that it will also provide opportunities to enrich our lives.
* The information contained herein is current as of November 2020.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.
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