Patent Application
20110192566
The present invention relates to thermal storage systems adapted to store energy in the ground, absorb the stored energy from the ground and transfer the stored energy into a thermally conductive wall structure.
Heating and cooling buildings consumes a large amount of energy. This is particularly the case in climates where there is a great disparity between maximum summer and minimum winter temperatures, as in much of North America, where it is necessary that buildings are cooled in the summer and heated in the winter.
Buildings are cooled and heated by a variety of means, including air conditioning units, electric heaters, wood stoves, forced air gas furnaces, and hot water or steam radiators. It is generally the case that a constant indoor temperature is desired depending on the external temperature. For example, room temperature (a temperature at which humans are generally accustomed for indoor living) is typically between 64-74° F. (approximately 18-23.5° C.), however local climate conditions may acclimatise people to higher or lower temperatures.
To minimize heat transfer between a building and its surrounding environment, various construction techniques have been developed which minimize the amount of energy required to maintain constant indoor temperatures. Examples of such techniques include designing and using building materials and insulation with high values of thermal resistance (also known as R-values), and employing air-flow heat exchangers which minimize the amount of heat lost to the external environment in the winter and reduce the amount of heat gained from the external environment in the summer.
Another way to improve the energy efficiency of a building is to make use of available geo-thermal energy. As is well known, the ground temperature below the frost line in much of North America is a relatively stable 55-56° F. on average (approximately 13° C.) throughout much of North America, ranging from around 41° F. (5° C.) in northern climates to about 71° F. (21.6° C.) in southern climates.
Ground-source heat pumps are one well known type of technology which take advantage of this physical phenomenon. Heat pumps typically have a series of heat exchanging coils buried in the ground below the frost line. In warm summer months, water can be cooled to the ground temperature when circulating through these coils. This cooled water can then be circulated in radiators located inside the building to cool the interior space, among other applications. In a similar way, a building can be heated in the winter by warming the water in the heat exchanging coils.
However, there has been a lack of construction technology specifically designed to take advantage of the fact that the ground surrounding and underlying a building can be used as a heat sink in the summer and a heat source in the winter.
Furthermore, there has also been a lack of construction technology adapted to store abundant heat energy in the summer for use as a heat source in the winter months.
Therefore, there is a need for building structures and techniques which reduce energy consumption by using the ground for heat storage in the summer such that the ground can be as a source of heat in the winter.
The present invention provides a thermal storage system for storing heat in the ground beneath a building such that the stored heat can be transferred to a thermally conductive ground engaging footing. Therefore, in at least one embodiment, the thermally conductive ground engaging footing can form the below grade foundation of a vertical wall structure that is contemplated to absorb the stored heat from the ground through the footing into the interior structure of the wall. This arrangement can reduce the heating costs of a building by raising the internal temperature of the vertical wall structure in the winter.
In at least one embodiment, the thermal storage system of the present invention can be used to cool the ground beneath a building by conducting heat from the vertical wall structure through a thermally conductive ground engaging footing into the ground. This arrangement can reduce the cooling costs of a building by lowering the internal temperature of the vertical wall structure in the summer.
In at least one embodiment of the present invention, there is provided a thermal storage system which includes a longitudinally extending ground engaging footing, the footing extending horizontally through the ground below the frost level and having an upper surface and a lower surface, pumping means configured to circulate a working fluid, a heat exchanger, the heat exchanger configured to transfer heat between the working fluid and the outside environment, the heat exchanger fluidly communicating with the pumping means, at least one supply outlet, the at least one supply outlet fluidly communicating with the heat exchanger, at least one length of pipe, the at least one length of pipe fluidly communicating with the supply input, the at least one length of pipe thermally communicating with the ground under the longitudinally extending ground engaging footing, at least one return inlet, the at least one return inlet fluidly communicating with the at least one pipe such as the pumping means circulates the working fluid through the heat exchanger to the at least one length of pipe by way of the supply outlet, the working fluid experiencing heat transfer with the at least one length of pipe, the at least one length of pipe experiencing heat transfer with the ground, the working fluid returning to the pumping means by way of the return inlet.
In at least one embodiment of the present invention, there is provided a method of storing heat in the ground beneath a building which includes the steps of burying at least one length of pipe in the ground below the frost level, the at least one length of pipe configured to receive a working fluid from a heat exchanger, the at least one pipe thermally communicating with the ground below the frost level, forming a longitudinally extending footing in the ground above the at least one length of pipe, supporting a vertical wall on the footing, the vertical wall extending upwardly from the footing to a selected height above grade, sheathing the vertical wall in insulation, lacing the interior of the vertical wall and the footing with thermally communicating heat conducting members, at least some of the heat conducting members extending outwardly from the footing into the ground a selected distance to facilitate heat transfer between the ground and the vertical wall.
Preferred embodiments of the present invention will now be described in greater detail and will be better understood when read in conjunction with the following drawings in which:
As is well known, the temperature inside the ground below the frost line is a relatively stable 55-56° F. on average (approximately 13° C.) throughout much of North America, ranging from around 41° F. (5° C.) in northern climates to about 71° F. (21.6° C.) in southern climates. This temperature is above the normal ambient atmospheric temperature during northern winters and below normal ambient atmospheric temperatures during the summer in most places. This delta temperature has therefore been previously used to effect a heat transfer that warms in the winter and cools in the summer. The heat transfer however has typically been accomplished using conventional heat exchangers that transfer heat from one fluid to another by means of direct thermal coupling or airflow.
The present invention seeks to store energy in the ground beneath a building and use this stored heat to heat a wall structure through direct thermal conduction. Alternatively, the present invention can be used to cool the ground beneath a building such that a wall structure can be cooled through direct thermal conduction with the cooled ground beneath the building.
With reference to
The ground 2 itself is the heat source for the present wall structure during the heating season so footing 15 is the primary thermal interface between the ground and wall 25 where the ground's energy is picked up.
Footing 15 will advantageously be positioned at least three feet below the local frost level, this level being the depth to which the ground will normally freeze in the coldest part of the winter, and is a poured concrete slab having a preselected transverse width of preferably at least 24 inches. Smaller widths are possible but testing shows that better results are obtained at 24 inches or greater. The footing's height will preferably be at least 8 inches but again this is variable. As will be appreciated, local building codes and engineering requirements will dictate the footing's minimum structural and dimensional requirements but the present invention may require that those minimums be exceeded.
The concrete for the footing will be gravel type having a minimum thermal conductivity of 2.0 W/mK. As will be known in the art, some concretes are not thermally conductive and the use of these for the footing is preferably avoided. Avoided concretes include lightweight, pumice powder, cellulose, isolation or slag concretes, all of which have significantly lower thermal conductivity. Applicant has found that the conductivity in the footing is increased using gravel having a 19 millimetre average particle size.
To increase heat transfer from the ground to the footing, the interface 5 between the two may optionally be laced with galvanized steel dowels 50. These can be laid in a cross-hatched pattern or linearly in the longitudinal direction of the footing on 16 inch centers, although other spacings are contemplated as well. Other patterns and configurations are possible, the idea simply being to facilitate heat transfer from the ground to the footing by means of these conductive elements. More effective means of promoting heat transfer from the ground to the footing are described below.
Within the footing itself, there will normally be reinforcing bar (rebar) in any event for strengthening the slab as necessary to meet local code and engineering requirements. Advantageously, the rebar will include a plurality of longitudinally extending continuous runs of steel 15M(#5) rebar 55. This rebar is normally located below the footing's horizontal center line as shown most clearly in
A continuous and longitudinally extending strip 57 of heat conducting material is positioned on the footing's upper surface 29 in the position shown most clearly in
The next element in the ground source pickup is a series of vertically oriented, longitudinally spaced apart dowels 52 that extend from a point in the footing close to but preferably not in contact with the ground/soil interface 5, vertically upwardly through the remainder of the footing and into the lower reaches of wall 25 as shown most clearly in
In the alternative to using the two sets of dowels 52 and 58, dowels 58 can be eliminated if dowels 52 are downwardly elongated to penetrate through the footing and into the ground to a predetermined depth, preferably at least 4 inches as shown in
The next element of the wall system is to provide a conductive path for the heat absorbed from the ground into wall 25 itself.
With reference to
The conductive elements of the grid are center line vertical conductors 80, horizontal center line conductors 85, off center vertical conductors 90, off center horizontal conductors 95 and horizontal continuity links 100.
Starting from the bottom of wall 25, off center conductors 90 extend upwardly from strip 57 to a point in wall 25 a selected vertical distance above grade. The lower end of each conductor is biased, welded or otherwise connected to strip 57 so the two are thermally connected for heat transfer purposes. Conductors 90 are located off center of the wall more towards its interior surface 24 to better isolate the conductors from the wall's cold outer surface 26 and any moisture that might penetrate the wall from the ground. Off center horizontal conductors 95 are tied or otherwise connected to vertical conductors by means of metal wire, clips or other means known in the art, the only requirement being that all intersections between the conductors be thermally conductive. As seen in
Above grade, vertical conductors 80 can be positioned along the wall's vertical center line with the horizontal conductors 85 connected thereto in the same manner described above with respect to conductors 90 and 95.
A thermally conductive continuity link 100 connects lower conductors 90/95 to upper conductors 80/85. The link can be made up of short sections of the same conductors used for conductors 80, 85, 90 and 95 that thermally connect the upper and lower conductor grids together for heat transfer therebetween.
The conductors in wall 25 can be lengths of 10M(#4) steel reinforcing bar connected together in a preferably minimum 16 inch on center grid in both the horizontal and vertical directions. As will be appreciated, the conductors can perform double duty as reinforcing for the wall itself in accordance with local building code requirements and engineering specifications.
As will be seen in
Unlike footing 15, wall 25 is preferably poured from low thermal conductivity concrete to minimize heat transfer from its warm side to its cold side. Again the concrete can be gravel concrete but using gravel having a 12 millimetre average diameter is preferred.
As mentioned above, the wall from footing 15 all the way to its top should be monolithically sheathed in insulation 40 so that there are no significant breaks, gaps or openings in the coverage. The insulation can be a foam type such as expanded polystyrene readily available from most building supply stores and which is manufactured in sheets. The foam insulation can be connected to the wall by means of adhesives, staples or any other means known in the art that are not thermally conductive. Whichever means are chosen should obviously minimize thermal conduction from the wall/insulation interface to the insulation's outer surface. For good results, the insulation on the wall's vertical surfaces should be minimum R9, and R25 along the wall's upper edge 27.
Any openings in wall 25 for doors, windows or other features should preferably be lined with slabs of foam or other equivalently insulative materials to prevent thermal loss around the opening edges. Equivalent materials can include for example the use of low expansion insulating foams injected into the peripheral gaps between the window/door and the wall openings to secure the windows/doors in place. The use of metal fasteners between the windows/doors and the concrete of wall 25 is preferably avoided to minimize thermal conduction.
Advantageously, the upper surface 29 of footing 15 on the side of interior wall surface 24 is also insulated for example by a piece of foam 31 (preferably minimum R8) to insulate the footing from the building's floor slab.
Wall 25 will itself extend from the building's footings 15 up to its eaves. It is preferable that wall 25 has minimal openings and penetrations as it is important to maintain as monolithic a construction as possible to maintain the integrity of the wall's thermal conductivity.
In operation, it has been found that a wall structure as described above conserves heat within the building and significantly reduces heat transfer from the inside to the outside in winter and from the outside to the inside in the cooling season. As will be appreciated, during the cooling season, the wall acts in reverse to its operation as described above in relation to the heating season and will conduct heat from above grade to the ground below grade.
With reference to
With reference to
The present invention also relates to a thermal storage system 200 that will now be described with reference to
Referring now to
The working fluid can be water, glycol, a mixture of glycol and water, or any other fluid that has suitable environmental and heat transfer properties for use in the present invention.
In the embodiment shown in
It is also contemplated that a single loop of piping could be buried beneath slab 300, and furthermore three or more loops of piping could also be employed in the present invention.
In one embodiment contemplated by the applicant, underground piping 202 is typically buried approximately 32″ beneath the lower surface 301 of slab 300, however underground piping 202 could be located deeper or shallower depending on climactic conditions, and soil thermal conductivity.
Underground piping 202 can be constructed of schedule 40 stainless steel tubing, however pipes of different thickness and constructed of different materials are also contemplated. For example, climactic conditions permitting, underground piping could be constructed of plastic or other metals, such as titanium, galvanized steel, cast iron or aluminum.
Conduit 204 has an inlet end 208 and an outlet end 209. Similarly, conduit 206 has an inlet end 210 and an outlet end 211. These ends are connected to a manifold 215 for the supply and return of working fluid from pump 220. Specifically, working fluid is pumped from pump 220 and enters conduits 204 and 206 through their inlet ends 208 and 210 respectively via manifold 215. After circulating through conduits 204 and 206, the working fluid leaves the conduits through their outlet ends 209 and 211, respectively, into manifold 215 where the return flows are combined for flow back to pump 220. In embodiments with more or less than two loops, there will be a corresponding number of supply inlets and outlets and an appropriately modified manifold.
In at least one embodiment, the working fluid is pumped through a heat exchanger 230 where the fluid can be either cooled or heated depending on the exterior ambient temperature and the availability of solar radiation. Heat exchanger can simply be a series of radiation absorbent plastic or metal pipes located on the roof of the building, or it could be a more sophisticated model including those which could recover waste heat emitted from the building and or sources from within the building. Heat exchanger 230 can be a solar collector or any other type of exchanger that is suitable for the use in connection with the present invention. Heat exchanger 230 can be an open loop or closed loop configuration.
The heat exchanger can be located either upstream or downstream from pump 220, such that it can receive either depressurized return working fluid before it is pressurized by pump 220 or it can receive pressurized working fluid after it has been pressurized by pump 220 depending on the thermodynamic requirements of the application.
Therefore, in one embodiment of the present invention, the working fluid is pressurized by pump 220 and supplied to heat exchanger 230 so that in the warm summer months, the heat exchanger transfers heat from the environment into the working fluid. Warmed working fluid is then circulated to underground piping 202 in the manner described above, causing the temperature of the soil in a heating zone 255 surrounding the piping within a radius 260 to rise as the warmed working fluid continuously circulates.
As will be seen, the radius 260 of heating zone 255 is selected so that from the mid point between pipes 204 and 206, the zone ideally reaches but does not significantly overlap footings 15 so that there is minimal heat transfer to the thermal walls during the cooling season. Obviously, it is not possible to precisely control the size of heating zone 255 due to fluctuations in solar radiation, soil type and density, the presence of ground water and other factors, but for any given geographic area, historical temperature and climatic records and soil measurements can be used to calculate the placement of pipes 204 and 206 with reasonable accuracy. In the example shown in
In other words, in western Canada, there are, on average, enough days having an above ground temperature in excess of ground temperature to heat a zone having a radius 260 of approximately 44″ inches. In geographic areas having more warm days, this radius will be greater and conversely, in colder climates having fewer warm days, the radius will be smaller.
If preferred, temperature sensors can be strategically placed to read soil temperatures and to transmit signals based on the temperatures to actuators to discontinue the circulation of working fluid if and when zone 255 begins to encroach on footings 15.
To contain the heat beneath the building during the heating season, it is preferred that slab 300, rather than being a typical reinforced poured concrete floor, is instead an insulating layer. To this end, slab 300 can be constructed in a number of ways that will be apparent to those skilled in the art. In a preferred embodiment, slab 300 consists of a layer of gravel e.g. 40 millimeter) compacted to a thickness equal to the thickness of footing 15, which in the example given above, is 8 inches, topped by a 4 inch thick covering 270 of expanded polystyrene foam insulation. Additional 2 inch thick strips 275 of EPS insulation can be embedded in the gravel directly above conduits 204 and 206, the width of the strips being selected to intersect the radius 260 of heating zone 255 at points 261 and 262. As will also be seen in
In this way the present invention provides a means for storing heat under building slab 300. Once the weather turns cooler in the fall and winter, this stored heat, which will migrate towards the cooler ground around footings 15, can be used to heat wall structures 10, offering a reduction in energy input for heating and associated reduction in energy costs during the winter months.
It is contemplated that at certain times of the year the system may not be as thermodynamically effective, as the temperature difference between the soil under the building and the ambient environmental temperature may be negligible. Therefore, in at least one embodiment, valve 214 is provided so that circulation of the working fluid can be slowed or halted when climactic conditions dictate or when system maintenance is necessary. The valve can include an outlet for example to replenish, drain or replace the working fluid. A user can monitor the system and when it is determined that the climactic conditions are not ideal for running the thermal storage system 200 in connection with the wall structure 10, the valve can be closed off. Furthermore, two valves can be provided, one upstream and one downstream of pump 220, so that the pump can be effectively “locked out” for maintenance or replacement.
In another embodiment, the present invention can be configured such that working fluid is glycol or a glycol-water mix that does not freeze in winter temperatures. In this embodiment, the system can be operated towards the end of the winter months to cool the soil beneath the building by circulating the working fluid to heat exchanger 230 when ambient environmental temperatures are colder than the soil temperature beneath the building. In this way, when working fluid passes through heat exchanger 230, heat will be transferred from the working fluid and dissipated by heat exchanger 230 so that the temperature of working fluid will decrease and the soil surrounding underground piping 202 will be cooled. In this arrangement, the cool ground can be used to cool wall structure 10 and reduce energy input for cooling and associated energy costs in the summer months.
By way of example, the thermal storage system of the present invention was tested using the following input parameters:
Based on the above input parameters, the skilled person in the art can now calculate the conductive heat transfer, the temperature loss from the input of the working fluid to the return of the working fluid, and the necessary mass flow rate for the system, using heat transfer and thermodynamic calculations known in the art, for proper configuration of the system, including the depth the conduits 202 are buried below the lower surface 301 of slab 300, and the horizontal spacing or inset of the conduits from the inner edge of footing 15.
Thermal storage system 200 can be retrofitted to an existing structure by trenching around its periphery for installation of conduits 204/206, or as the case might be, and an insulating barrier over the conduits to perform the function of slab 300 and EPS layer 270.
The above-described embodiments of the present invention are meant to be illustrative of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications, which would be readily apparent to one skilled in the art, are intended to be within the scope of the present invention. The only limitations to the scope of the present invention are set out in the following appended claims.
