SE408087B - SEE THAT IN A GROUND BODY STORES THERMAL ENERGY - Google Patents

Description

.. ma: a.- 7?1:0'71+8~0 When using wind energy for local heating, there is a similar need for heat storage. In many process industries, large amounts of heat are obtained at relatively low temperatures, which could be used for local heating, for example. The lack of an economic possibility to be able to store this heat until a heat demand exists means that large amounts of energy are wasted.

For example, in combined heat and power plants where large variations in electricity demand make rational use of heat production impossible, a heat storage option would be desirable.

Incineration of waste in combination with a cheap heat storage method would involve the utilization of an energy resource that is not currently used.

A cheap and simple energy storage method that can be used on a small scale also encourages individuals to make rational use of waste materials that would otherwise go unused.

Today, several methods are used to store heat. Different materials are used for heat storage. One method utilizes the specific heat of the materials by heating them. Other methods utilize the heat of fusion or heat of vaporization of a material by adding heat at the melting or boiling point, respectively. Another method uses the energy exchange during crystal transformation in certain materials. Some methods are used today in ready-made systems, other methods are in a development stage.

Which method is used depends on the desired operating temperature range, compactness, acceptable heat leakage, extractable power per weight/volume, installation costs, etc.

What the methods presented above have in common is that none are intended or suitable for storage methods up to parts of a year; the storage time for the best methods is a maximum of a few weeks.

In an application example, water is used as a storage medium when the energy is obtained from a solar collector. The stored heat energy is to be used for heating a residential property. If radiated solar heat obtained during a day is to be used for one day's consumption, and the temperature range is allowed to be 60-95°C, a well-insulated water tank of 2-3 m3 is required. Heating during a day with little sun must then be done with additional heat in some form. If one wants to reduce this inconvenience, one can install a larger solar collector and a larger heat storage unit. However, the cost of such an ambition is unjustifiable at today's energy prices. 7710748-0 A more interesting heat storage method has also been proposed recently and consists in arranging a ground body that is in direct thermal connection with the surrounding ground, as stated in the introduction. The channels arranged in the ground body can, according to one proposal, consist of boreholes in the ground, in each of which a pipe loop with a flowing fluid is immersed. According to another proposal, the channels consist of shafts blasted into the rock according to a certain pattern and connected by boreholes. In both cases, high temperatures of the fluid flowing in the channels are required (normal radiator temperature is at least 50°C) and a complicated control system to control the input and output of the thermal energy. Due to the high temperatures, the losses to the surrounding ground and in the fluid cycle are very large, since, for example, the surrounding ground has a significantly lower temperature (in Stockholm 800). If solar collectors are used, these have poor efficiency at high temperatures.

'These disadvantages are in practice so considerable that the method has not been able to be applied under realistic and economically justifiable conditions.

What is desired is a heat storage device that can be made without excessive costs: with a sufficiently high storage capacity, with losses that can be economically compensated for, for example with solar collector surface area, and with simple technology and simple materials.

This has been achieved according to the invention by arranging the channels in such a number and with such a dimensioning and distribution depending on the calculated supply and withdrawal of thermal energy over a preferably long period of time, for example one year, that a boundary surface surrounding the ground body at a distance a sz _ Q, where a = -E- , whereby ay is the frequency of the temperature variation 'Y P (periodic) I cp = the specific heat of the ground body JL = the thermal conductivity of the ground body and af the density of the ground body, from points on the outermost channels calculated in all directions outwards from the ground body, obtains a maximum temperature of '(71 07% -0 u of the order of 35°C and a variation due to supply and withdrawal of energy over the period of time of at most l0°C. _ The invention is based entirely on the fundamental difference in comparison with the previously proposed above-related methods consisting in the fact that such a low temperature of the boundary surface surrounding the ground body as approx. 35°C was used. I When using solar collectors, it is advantageous if the fluid leaving the solar collectors has a temperature that is limited to a maximum of ß5°C, preferably 35°C, whereby the solar collectors can be made extremely simple in their construction and still have a very high efficiency, even higher than the most sophisticated, focusing solar collectors that operate with temperatures of up to 100°C, of the fluid.

Especially in combination with low temperature of the heat-emitting devices, a high overall efficiency is obtained.

The method according to the invention thus achieves, among other things, the following advantages: a) The capacity can be made as large as required without high costs, b) the losses due to heat leakage are so small that they can be economically compensated for, for example, by increasing the solar collector surface area, c) today's technology is applicable and no complicated components are required, I 7 7 d) simple solar collectors can be used with good efficiency, e) no major thermal stresses or fatigue phenomena arise in the soil body, and f) the maximum excess temperature of the soil body is so low that ecological damage does not occur.

Of course, the heat transfer system must be dimensioned so that the required amounts of heat can be transferred from the fluid to the room at a temperature of the fluid that does not differ from the room's temperature by more than 10°C. The soil layer consists primarily of an inner zone Zl (see Fig. 3a and Sh), which is provided with channels or conductor paths for supplying heat. The boundary surface Y1 for this zone consists of the surface that encloses the active conductor paths in the ground. When calculating the storage capacity of the soil layer, the zone defined above can be assumed to follow completely the temperature variations of the conductor paths, provided that each volume element in the body is at most the distance sl from one of the conductor paths. 'where a = 7710748-6 From Jacob, Heat Transfer, Sixth printing, March 1958, page 303, it is obtained that and Q)= the frequency of the temperature variation (periodic- Ä.

I'°P disk), where Ä_= the thermal conductivity of the soil body I = the density of the soil body and the specific heat of the soil body.

°P The distance between two adjacent conductor paths should therefore be less than 2 sl. .In the event of periodic variation of the temperature in the boundary surface of the zone, heat will migrate out or be withdrawn from the surrounding ground. The heat that is thus cyclically supplied or withdrawn from the surrounding ground can be written according to _ Jacob: Heat Transfer, page 293, Q =Yl.20a.¿e. -åï where 29a is the temperature variation (see fig. 30) .ß . cp lll This amount of heat Q can be stored in an outer zone Z2 in the soil layer, which completely participates in (follows) the temperature variation 2 a of the surface Ylz.

The volume of this outer zone Z2 can be written Y1 . s2, where sz can be interpreted as the "equivalent penetration depth" in the soil outside zone Zl. The volume of the soil layer can thus be considered as the sum of the zones Zl and Z2 and with a boundary surface Y2, which is at a distance sz from the surface Yl. 1 Alitsågälier Q=Y.26a.,,(.-Ví=s2.srl.29a.f.cp whereby sz = \/ å .

We thus find that sl = sz .

To achieve optimal utilization of the soil mass working in the heat storage process, the channels are suitably distributed so that each mass element participating in the process has a maximum distance to a channel of about 1 meter for very water-rich soil to about 3 meters for very water-poor soil or bedrock, such as granite. 7710748-0 To optimize the possibility for the soil mass working in the heat storage process to utilize the maximum available effect, the total available surface area of the channels can be given a suitable size by adapting the effective line length and line diameter. _ The required channels can be achieved particularly easily if the method according to the invention has the characteristics specified in claims 5 or 6.

The invention and the advantages achieved thereby are explained in more detail in the following with reference to the attached drawings, in which Figure 1 shows an exemplary embodiment of a simple conductor path achieved by means of a borehole, Figure 2 shows an example of a soil body participating in heat storage under a villa, Figures 3a and 3b show a soil body in two projections, and Figure 3c shows the area of temperature variations in a section through the soil body.

Figure 1 shows a channel or conduit arranged in a body of land 1 which has been produced by drilling, in addition to a heat-insulated supply line of 3 m length, a hole 2, for example 10 meters deep, with a diameter of for example 2.5 cm, which borehole is lined with a "sock" 3 of for example aluminum foil, in which a concentric supply pipe N is placed. The lining 3, which has been pressed into contact with the walls of the borehole 2 by for example an overpressure shock, is sealingly connected to a connecting pipe 5, which, like the pipe 4, is connected to a pipeline 5, over which the pipes 3,4 together with a device 7 for supplying heat, for example a solar collector, and a device 8 for releasing heat, for example radiators, form a closed circuit for a flowing fluid, for example water. In order to prevent the temperature on the ground surface from being significantly affected, the upper part of the pipe is insulated for approximately the distance S2.

The boreholes 2 are arranged with intervals of the order of magnitude of at most 2.s, the value of which depends on the soil type and is given below: v 77107lr8'0 Soil type Distance 2 sl (cycle1=1 year)2 32 Granite 6.H m Sand U.6 m Moraine _ 5.U m Clay ' ' 3.5 m Mud 2.0 m Water 1.8 m Figure 2 shows a normal villa 10 with length x width dimensions x 8 m built on bedrock and with an annual energy requirement of 26,000 kWh, on which an essentially horizontal solar collector 11 of H0 m2 is thought to be built into the roof. From this it can be calculated that for 100% coverage of the annual requirement with solar energy a soil body 12 according to the invention with a volume of 2300 m3 is required. This is easily achieved by using two rows of boreholes 10 m deep and spaced about 6 m apart (2sl = 6 m). The boreholes do not need to be vertical, of course, but can be inclined if necessary.

The soil body (granite) extends approximately 3 m out from the two rows of boreholes both outwards to the sides and downwards (and upwards if the upper parts of the boreholes were insulated) and comprises a volume of >2300 m3.

Figure 3a shows a vertical section through a soil body l2 formed by four rows of vertical channels or conductor paths 2.

A limiting surface Y1 surrounds the channels 2 and passes through the outermost channels and their end surfaces. The distances between the channels and the zone enclosed by the limiting surface Y1 are at most the distance sl. The zone is designated Z1. A zone Z2 extends, which has the extent Around the zone Z1 These zones and surfaces are shown upwards to a limiting surface Y2. from in Figure 3b.

Figure 3c shows the temperature distribution within the extension 12' of the soil body in a horizontal plane and the temperature swing with the size 2Ûa when inputting and extracting heat energy to and from the soil layer, respectively. The position of the limiting surfaces Y1 and Y2 shown in Fig. 3a and b' are marked in Figure 3c as well as the temperature distribution outside the outer limiting surface Y2. ¿v?1ov4s-o To increase the temperature of the soil body according to the invention, for example to 25°C and 30°C, a relatively large amount of energy is required during an initial stage, which can be obtained from, for example, temporary solar collectors set up on the construction site. Other temporary heat sources are also conceivable. The cost of this initial derivation of the soil body can be considered an investment.

In a similar way to a normal villa, an apartment building can be equipped with, for example, solar collectors and a ground source, which can work against a low-temperature heating system in the house. Covering 100% of the heating needs in this way, even in apartment buildings, can of course provide major economic benefits.

In many cases, and especially in countries with a ground temperature of up to 20°C, it may be appropriate to use the same principle, alternatively or in addition, to arrange a cooler ground body with a temperature of about -15°C to keep a room cool that has been exposed to unwanted heat for a long time.

The ground storage method according to the invention can also be used for temperature control of swimming pools.

The invention is generally applicable to varying supply of thermal energy from solar collectors, wind generators (via mechanical water brakes or electric heating), waste heat and varying or constant withdrawal of thermal energy, but also to constant supply of thermal energy and varying withdrawal, which may momentarily be greater than the energy supplied.

The dimensioning of the soil body and the distribution of the channels can in some cases require complicated calculations, which are of course greatly facilitated if computer processing is used.

The invention is of course not limited to the embodiments shown here, but numerous modifications are possible within the scope of the invention defined in the claims.

Claims (6)

.vyf fi - 9 7710748-0 P A T E N T K R A V A method of storing thermal energy in a soil body which is in direct thermal communication with the surrounding soil, which from heat-absorbing devices, such as solar collectors, is supplied to the soil body via a plurality of channels arranged in the soil body by circulating a fluid in a circuit containing the channels and the heat-absorbing devices, and extracting stored thermal energy from the ground body by means of the fluid for tempering an object, such as a building, by circulating the fluid in a circuit containing the channels and heat-emitting devices, due to the fact that the channels are arranged in such a number - and with such a dimensioning and distribution depending on the calculated supply and extraction of thermal energy for a preferably long period of time, for example one year, that a ground body surrounding boundary surface at the distance 7 \ 3.cp wherein LJ is the frequency of the temperature variation (periodic) where a = cp specific heat for the ground body '7L thermal conductivity the density of the soil body _3 = the density of the soil body, from points on the outermost channels calculated in all directions outwards from the soil body, obtains a maximum temperature of the order of 35 ° C and a variation of energy supply and withdrawal during the time period not exceeding 10 ° C. 2. A method according to claim 1 for tempering a building, characterized in that the fluid is supplied to the heat-emitting devices at a temperature which does not differ from the intended room temperature of the building by more than 10 ° C, preferably 5 ° C. 3. A method according to claim 1 or 2, wherein solar collectors are used as heat-absorbing devices, characterized in that the fluid emanating from the solar collectors has a temperature which is limited to a maximum of HSOC, preferably 3590. "Ära _ 7710748-0 1 ° 4. A method according to any one of claims 1-3, characterized in that in order to achieve optimal utilization of the storage process, the channels are distributed so that each mass element participating in the process has a maximum distance to a channel equal to the distance s. 5. A method according to any one of claims 1-4, characterized in that the channels in the ground body are provided by drilling a number of holes from the ground surface and inserting a liner, preferably of metal with the required compressive strength in each borehole, in which a concentrically, open pipes are arranged at the bottom of the borehole, and the pipes and liners are connected with pipelines to form at least one closed circuit containing the heat-emitting and heat-absorbing devices. A method according to any one of claims 1-M, wherein the soil has a soft nature, characterized in that the channels in the soil body are produced by pressing into the soil body a number of tubes closed at the end of the depression and arranging in each tube a concentric, at the closed end of the tube open inner tube, together with the outer and inner tubes are connected with pipelines to form at least one closed circuit containing the heat emitting and heat absorbing devices. MENTION PUBLICATIONS: