Aqua thermal: heat sources and energy saving

Site last update July 15th_2021

Carbon resources, geothermal resources, nuclear fission, waterfalls, wind and sunshine are used for various forms of energy production, including heating public and private buildings. These resources have respective advantages and disadvantages recalled below. There are, however, other means for producing heating presenting a large number of advantages with very few disadvantages. These means use the heat contained in our immediate environment. They are therefore accessible to most of us even to the ones living in collective buildings.

Among the carbon resources, the raw material is in the form of coal, wood, plants, petroleum, natural gas, coal gas or shale gas. These raw materials provide significant amounts of heat during their combustion which can be used directly for heating or indirectly for the production of mechanical or electrical energy. These energies have major disadvantages due to their method of production, their transport, their transformation as well as the production of pollutants, toxic gases and carbon dioxide all harmful to individuals and the environment. These resources nevertheless remain widely used due to their ease of storage in the short, medium and long term. However, these energies are not sustainable, apart from carbon surface production (wood, plants) considered to be semi-renewable. The extinction of fossil fuels should occur in 100 to 300 years (very large uncertainty according to experts given the multiplicity of parameters). Note the very special case of carbohydrate resources constituting very large reserves, located at very great depths and whose exploitation has not yet started due to its difficulty of exploitation.

Regarding geothermal resources, it may be found at great depth, reservoirs of water at a very high temperature and pressure, these two parameters increasing generally notably with the reservoir depth. For example, at a depth of 4 000 metres, water is often around 100 ° C. When this water is highly pressurized, it is possible to recover, on the surface, energy in a mechanical form using single-phase turbines with prior separation of the liquid and gaseous phases otherwise using two-phase turbines designed for simultaneous expansion of the two phases (see the section on two-phase turbines). In other cases, the pressure is not sufficiently high to boost the hot water towards the surface requiring pumping of the water with submerged pumps. This hot water may later be used for the heating of large residential areas. Geothermal resources are only available in certain areas on the earth. In addition, this technology is quite expensive given the importance of the drilling at a great depth (several thousands meters), corrosion and erosion caused by effluents and the requirement also for re injection wells. More recently, it has been observed that the drilling of very long well may enhance the risk for seismicity. This form of energy is generally considered to be sustainable, the heat coming indirectly from the Earth’s magma. However, some geothermal sources are considered non renewable, when heat extraction occurs at a higher rate than that of heat supply. It has also been observed that geothermal sources contain lithium which may be of a great value at the present time where the industry is looking for this material for the manufacturing process of electric equipment, particularly, batteries.

Nuclear fission provides a large amount of energy which is free from toxic or greenhouse gases. However, this form of energy presents some environmental risks linked to the design or the operating mode of certain power plants, to the terrorist risk or to very occasional events like earthquakes or plane crashes. In addition, the storage of nuclear waste is the subject of some concerns and reflections which have not yet come to an end. Note that there is some hope regarding the treatment of very long-lived and highly radioactive waste that could be treated with the latest generation of pulsed laser (pulse of a billionth of a millionth of a second). This would be the case of neptunium, highly radioactive with a half-life of 2 million years which could be converted after a long chain of transformation into ruthenium and cesium, two stable atoms. On this subject, see the work of the 2018 Nobel Prize in physics, Gérard Amouroux.

It will be many years before nuclear fusion, a nuclear energy causing very few issues in terms of waste management, becomes available to the inhabitants of this planet.

Regarding hydroelectric power, one must benefit from a strong drop with a large water reserve at a high level to be able to collect mechanical energy (potential and kinetic energy conversion) at a lower level, through a water turbine, this energy being generally converted into electricity. This relatively clean energy is considered renewable however, it is seasonal and available in limited regions on the earth. In addition, it is expensive to produce (construction of dams and distribution of electricity) and presents several drawbacks for flora and fauna. The hydroelectric infrastructure provides a very strong advantage considering its capability to store the energy produced by the intermittent ones (wind and solar).

Wind energy although in constant development presents some advantages (green energy, sustainability) but also some disadvantages: huge foundations, irregularity of the wind, intermittence, very low average efficiency, need for energy storage, visual aspect, complexity in energy distribution, significant investment and impact on the greenhouse effect relating to the production of the elements linked to this technology.

Solar energy, also in constant development, presents several advantages (green energy, durability, continuous production cost reduction, continuous improvement in efficiency, flexible and transparent materials) and also some drawbacks: limited availability of raw materials in the future, irregularity of sunshine, seasonal effect, very low efficiency, installation on very large surfaces, very long time for money return, need for energy storage, energy distribution and impact on the greenhouse issue regarding the manufacturing of the equipment related to this technology.

There are many other forms of energy production including tidal energy, wave energy (at sea and seashore), energy from ocean streams, energy linked to the ocean thermal gradient (exploited when the sea is very hot on the surface and cold at depth for the activation of a thermal cycle), the osmotic energy that it is possible to produce in the estuaries of large rivers linked to the difference in salinity between the fresh water of the rivers and the salt water of the oceans.

The heating of buildings could also be carried out from the heating sources which constitute our immediate environment: water, ground and air. Despite of it present existence, it should be produced at a much more greater scale to provide considerable benefits.

Aquathermy is a source of energy still very little used around the world. It consists in extracting the heat contained into ground water, a river, from the sea shore or in a lake, through a closed thermodynamic cycle activated by a heat pump in order to transfer it to a volume to be heated. The operation of this cycle comprises four stages: firstly, a refrigerant gas is brought to a relatively high temperature during a polytropic compression. Secondly, the gas is sent to a first heat exchanger (a condenser) transferring part of the heat conveyed by this gas to a volume to be heated (hot source). Thirdly, at the outlet of the condenser, the partially liquefied gas is let down through a valve until a pressure level corresponding to the inlet pressure of the compressor transforming the residual gas into liquid. In a final stage, this liquid passes through a second heat exchanger (an evaporator) taking heat from an external volume (cold source) and transforming this liquid back into gas. The gas is again compressed through the compressor in order to start a new thermodynamic cycle.

During a cycle, the heat released to the hot source by the condenser is equal to the sum of the heats absorbed by the evaporator and generated by the compression of the gas. The coefficient of performance of the thermodynamic cycle is defined by the ratio between the heat released by the condenser and that provided by the machine driving the compressor. In most situations, the drive machine is an electric motor, however, in some specific cases it may be a thermal engine. In this last situation, the heat of the combustion gases (released into the atmosphere) may be added to the heat provided by the compression of the refrigerant gas.

It appears from this definition that when the heat released by the condenser is close to the heat absorbed by the evaporator, the heat generated by the compressor is close to 0, that is to say that the compression ratio is close to 1 (compressor outlet pressure and temperature are close to the inlet ones) and that the energy absorbed by the compressor is close to zero. In this case, the coefficient of performance tends to infinity. On the contrary, when the heat released by the condenser is close to that provided by the compressor (or the drive machine), that is to say that the heat brought by the evaporator is close to 0, the compression ratio is very high (compressor outlet pressure and temperature much higher than inlet ones). In this case, the coefficient of performance tends towards 1.

The term pumping is identified with heat exchanges between two temperature levels where, paradoxically, heat is taken from a cold source to be transferred to a hot source. It is obvious that this phenomenon is not natural and that it is somewhat costly in energy. Thus pumping will require all the more energy as the difference between the temperatures will be large (high compression ratio) and as the amount of heat exchanged will be large (flow of refrigerant gas circulating in the thermodynamic loop high). The relationship between the pressure and temperature variations at the inlet and outlet of the compressor is given by the polytropic compression law (To / Ti) = (Po / Pi) Exp M where M = (k-1) / k / Eta where To, Ti, Po and Pi are the ABSOLUTE temperatures and pressures at the outlet (hot source) and at the inlet (cold source) of the compressor, M, k and Eta are, respectively, the polytropic coefficient, the isentropic coefficient and the polytropic efficiency of the compression.

These comments highlight the importance of a good selection of the hot and cold sources. The cold source must be provided, when it is possible, with a high temperature. Therefore, there is a strong benefit in taking the heat from ground water at a few meters below the soil surface where the temperature in winter is close to 15 ° C rather than the heat in the air at a temperature below 0 ° C. The hot source must be designed with a low operating temperature, hence there is a strong benefit in using heating devices with a large surface (underfloor heating or radiators of the wall dimension) which allow heat transfer at a relatively low operating temperature of 25 to 30 ° C, unlike to small area central heating radiators which require a high operating temperature of around 60 ° C.

Although there is a very wide variety of heat pump operating situations, it is possible to give an average value of coefficient of performance. This value would be of the order of 4 meaning that in comparison with a strictly electrical heating, a heat pump consumes 4 times less energy for the same production of heat.

For several centuries, man has settled near rivers, streams, lakes or on the edge of estuaries or seas for multiple reasons: having water for crops, fishing, water mills, leather tanning or the transport of goods. As a result, today the majority of dwellings are located within short distance of large volumes of water that could be used for aquathermia.

A small river like the Orge (Department of Essonne – South of Paris) could provide a power of 4 MW for the heating of several buildings on the basis of a water flow of 1 m³ / s and a lowering of the temperature of 1 ° C. This slight drop in temperature has no consequence for the flora and fauna of the river, considering that after a few hundred meters, the water regains its initial temperature in a constant heat exchange with the soil and the ground. Often, below the river bed, a ground water stream is established in constant exchange with the river flowing on the surface, making it easier to restore the temperature of the river even more quickly. It is also preferable to take the heat directly from the ground water often located only a few meters below the river. It should be noted that the river in question has an average flow of 3 m³ / s in winter reaching 5 m³ / s during heavy rains. It is therefore a calorific power greater than 10 MW which could be taken locally, permanently, during the winter period in this small river. Along the river it would be possible to capture this energy level several times as long as the distance is slightly greater than a few kilometres.

In an urban complex comprising apartments and houses, the average heat consumption is estimated at 10 kW per dwelling. It is therefore a set of 1 000 apartment-houses which could be heated by a small river like the Orge.

Considerably more heat could be extracted from the large rivers. At St Nazaire, the flow rate of the Loire river is 930 m³ / s allowing a heat production of 3 720 MW by lowering its temperature by only 1 ° C. Given the permanent heat exchange with the ground (underground), it is probably more than 40 Giga Watt that could be extracted from the Loire (1 000 km) and its tributaries (Allier, Cher, Indre, Creuse and many others).

If we consider that the flow of the Rhine is 2 000 m³ / s in Strasbourg and that the flow of the Rhône is 1 700 m³ / s in Beaucaire, it is probably more than 200 Giga Watt that we could extract on the French territory of rivers and their tributaries. By way of reference, the Chinon nuclear power plant produces 3 600 MW of electricity (four 900 MW units), to which must be added 1 000 to 1 500 MW of heat rejection in the Loire. This reference to nuclear power plants leads to at least three comments:

– the extractable heat from rivers and streams is much greater than that produced by nuclear power plants. There is here a reflection to bring on the future development of nuclear power plants but also on the needs in fossil energies in areas in proximity of rivers or streams.

– if aquathermy consumes electrical energy for its operation, it is only the equivalent of a quarter of the heat production provided by a total electrical heating.

the rejection of heat in the rivers of nuclear power plants being of the order of a third or a quarter of their electrical production, the development of aquathermy would play a regulating role in this thermal pollution