EP0239573A1 - Method of recovering ice formation energy - Google Patents

Abstract

A heat transfer fluid is circulated through an air permeable screen, in order to cool said screen. Ice then forms on the screen surface due to the humidity of the air. The circulation of the fluid is periodically interrupted while air can pass through the screen covered with ice so as to accelerate the sublimation of the ice on the surface covered with ice of the screen under the effect of air. crossing the latter.

Description

METHOD OF RECOVERING ICE FORMATION ENERGY

The present invention relates to a method of recovering the energy made available when water freezes. The wind or - more accurately - the air masses which by movement in the open air form what we call the wind, contain heat energy emanating from the sun radiation. Such energy can be recovered by means of collectors, so-called heat receivers, for heat pump systems. This is explained in more detail in the Swedish patent application No. 8201039-8.

In such heat exchange between a heat-carrying fluid, e.g. water, and the heat flow passing by, which we call the wind, large energy quantities are recovered. These are utilized in the most advantageous way in combination with a heat pump wherein the heat content also of relatively cold air provides an energy supply. The power then recovered is related inter alia to the wind velocity and relative air humidity. The humidity of the air in that case is of interest, because a higher humidity increases the k value, i.e. provides more W per C and fin surface, and also because a higher density provides a larger heat content of the air, which thus can be heat exchanged. At humid weather, mist or fog, the moisture of the air which principally consists of steam, will condense, also the phase conversion energy being utilized providing a large energy supply.

At cold weather, the condensed water will freeze to ice on the surface of the collector and a further phase conversion then can be utilized. When water freezes, 336 kJ/kg water are made available, which should be conferred with about 4.2 kJ/kg that can be recovered when lowering the temperature of water by 1 C. The heat receivers which operate with the outside air as the heat source, are not constructed such that the ice formation energy provides an energy supply, because the ice must be removed rapidly by thermal de-icing. The energy that possibly has been received at the ice formation, then will be consumed for the removal of the ice from the surface of the receiver. Otherwise the admission of the air will be obstructed and the power delivery will be reduced.

Thus, the porosity of the collector screen is large and the admission of the air is not obstructed notwithstanding heavy ice formation on the several surfaces of the collector. Accordingly, heat will be exchanged through an ice layer, and the ice formation energy of 336 kJ/kg at the same time can be recovered. At low air temperatures but high relative air humidity, which is a common operational condition e.g. during the night, the temperature in the zone where the ice is being formed, will be 0 C, i.e. higher than in case no ice formation had occurred. In this manner the ice formation will supply energy to the brine liquid or the fluid circulated in the collector screen. By the arrangement the heat receiver therefore can absorb the heat energy at a higher temperature than that corresponding solely to the air temperature. If e.g. the air temperature is -10 C and the relative air humidity is 95%, the brine temperature will be over the air temperature due to the energy supplied from the ice formation energy. Additionally, an ice collector body is a good receiver of radiated heat, direct sun radiation, and so on.

The collector will not need specific de-icing, which is a great advantage as compared with the existing fin batteries or fan coolers. Due to the large area the air flows will always be admitted. During the shutdown periods of the heat pump, which comprise about 50% of the time at normal operation, the ice crystals will disappear and make room for new icing when the heat pump restarts.

If the tmperature of the outside air happens to be low, the ice crystals will not melt to water. On the contrary, they will sublime, i.e. they will evaporate without intervening melting. Then, the enlargement of the surface caused by the icing will be of great importance. The sublimation proceeds more rapidly when the surface is larger.

The sublimation which requires energy, takes place more intensely when the system is not operating, i.e. during the shutdown period when also the brine pump has come to a stop. The sublimation heat which is the melting heat + evaporation heat, therefore is taken from the passing air flow which is constant or in any case will not be affected by the shutdown periods. In this manner, the collector is active although the system has come to a stop. The generated cold thus does not affect the heat in the system; it is removed by the air flow, i.e. the wind power. Instead of de-icing in the strict meaning thereof a volatilization will take place. When the heat pump restarts and absorbs heat from the collector screen there will again be space for repeated ice formation.

An interaction between ice formation and sublimation thus provides heat energy for the collector which can operate at a considerably higher level than that corresponding to the air temperature of the open air. This contributes to an increase of the heat factor of the heat pump system, but above all provides the possibility of absorbing heat from the open air at far lower outside temperatures than is possible when using collectors of today. Since water is always available in dwelling-houses and most of such objects as are to be heated, meteorologically and geologically limiting factors for the use of heat pumps will be cancelled when the invention comes into use. In order that an effective ice formation or rather frosting shall take place moisture is always required. For the good function of the invention a certain moisture is required. If there is no moisture, the system will provide a good heat absorption also at bad operational conditions, such as low outside temperatures At low temperatures water is therefore sprayed through atomizing nozzles directly onto the collector body while the heat pump is operating. During the shutdown periods the water flow ceases and the ice will sublime. This is an extraordinarily effective method of utilizing the energy of water.

The heat pump systems operating with the outside air as the heat source, are dependent on de-icing means already at about +4 C due to the fact that the collector is clogged by ice which accordingly excludes the air. At an air temperature of about -5 C, so much energy is consumed for such de-icing by means of hot gas or hot water that it is no longer worthwhile to operate the heat pump. Other kinds of energy then have to guarantee the heat supply, e.g. an electric element or an oil-heated furnace. In everyday speech, the energy thus supplied is called peak energy. This condition is very disadvantageous for the community, because the network for distributing electric energy will be overloaded when the outside temperature decreases and the electric elements are energized.

Considering these circumstances a universal heat pump system is to be preferred. Heat pumps with underground water as the heat source are considered to provide such a system. Then, water is pumped through bores to the evaporator where 4-5°C can be recovered before the water is again returned to the same depth through another bore.

When 1000 1 of water are heat exchanged there can be recovered 1000 x 4.2 kJ = 4200 kJ per °C. If 5°C are recovered then accordingly 21000 kJ are obtained. The costs for the underground water arrangement are relatively high, however, and the electric energy supplied for the operation of the pump is substantial. According to the invention, the same amount of energy 21000 kJ will be recovered when 62.5 1 of water is sprayed over the collector surface and is allowed to freeze to ice there. As mentioned, 336 kJ/kg is made available, i.e. 62.5 1 x 336 kJ = 21000 kJ. This relatively small amount of water preferably is taken directly from the tap water network which holds about 7-8 C. During the winter season there is a rich supply of tap water.

The power required for heating a house may comprise about 8 kW. This power can be obtained by allowing 62.5 1 of water to freeze so as to make 5800 W available for the heat pump system which supplies about 3000 W from the compressor heat. Alternatively, an electric element could be used for supplying the totally required power about ^■ kW.

According to the invention, the tap water shall guarantee the total peak energy, i.e. 5.8 kW. Then, 62.5 1 are required at a price of about SEK 0.006/1 (about SEK 5.50/m³) at a total cost of SEK 0.38 for the 5.8 kW mentioned.

The price of 1 kW generated by means of the invention will be SEK 0.06. Today the price of 1 kW electric energy which is the most common peak energy, is about SEK 0.30. It can also be said that the value of the usual water used according to the invention, has increased from SEK 0.006/1 to SEK 0.028 or by 466%.

Today, lakes and streams are used as heat sources for heat pumps, the water being cooled directly in large so-called spray evaporators. Then, large water flows are involved the temperature of which is decreased only some C. In the winter time such plants must be operated at reduced power - the usual lake water temperature in the winter time is +1 to +2 C - and there is a great risk of ice formation in the evaporator of the heat pump. Systems are also available wherein the municipal tap water is heat-exchanged and between 4 and 5 C are recovered therefrom, the tap water then being discharged into surface water conduits. Such a system consumes large amounts of tap water at cold weather. There are also ice heat pumps wherein the energy is recovered by freezing lake water. The spray evaporator is replaced by an ice machine. The ice produced is scraped mechanically from the cooling surfaces. The ice flakes produced are pumped back into the lake and therefore other interests than the energy economy can be involved when accomplishing such plants.

The purpose of the invention is to make the wind screens according to the Swedish patent application No. 8000488-0 and the Swedish patent application No. 8201039-8 more effective by utilizing the heat energy available in the air which is heated close to the outside surface of a building body partly by the building body collecting large amounts of incident sun energy and partly by heat leaking from the interior of the building body in spite of an effective insulation. However, it should also be possible to apply the invention to constructions which are separate from buildings, e.g. pillars or towers, poles, and so on. For this purpose the method of the invention has obtained the characteristics appearing from claim 1. The method of the invention is applied most favourably in combination with a heat pump system, the heat content being utilized most effectively as a consequence thereof. In order to explain the invention in more detail it will be described below with reference to embodiments shown in the accompanying drawings in which

FIG. 1 is a fragmentary side view of the screen,

FIG. 2 is a fragmentary side view of the screen with ice formation in different stages,

FIG. 3 is a fragmentary side view of the screen with heavy ice formation but with remaining apertures (porosity) for the passage of the air flow through the screen, FIG. 4 is a side view of the screen from the outside thereof at a building body in combination with a heat pump system,

FIG. 5 is a plan view of a freely located screen construction formed as a pillar wherein the screens are interconnected in parallel, and

FIG. 6 is a fragmentary cross-sectional view of the screen with supporting poles.

In FIG. 1, the air-permeable screen is shown. The apertures allowing passage of the air flow 10 are large in relation to the parts 11 and 12 of the construction having a braking effect on the air flow. The screen is made of aluminium or another good heat conductor. In that way the screen heat-exchanges the wind passing therethrough as described in the Swedish patent application No. 8201039-8. The fin surface of the screen shown in FIG. 1 is about 2.5 times larger than the area.

Channels for the circulating heat-carrying fluid are provided in the parallel parts 11, and the interconnecting fins 12 are surface enlargers. In FIG. 2, the screen is shown in different ice-forming stages wherein 14 are the zones which are initially covered by ice, the zones 15 then being covered by energy-supplying ice. However, at the same time the air flow is admitted and can supply the thermal energy thereof to the different fin surfaces of the screen. A larger relative air humidity provides a larger energy supply and so does an increasing wind velocity (cfr. the diagram below). In the operational condition illustrated, the fins of the screen are still free from ice and thus can exchange heat directly to the metal. Then, it can be assumed that the surface of the screen has a temperature of at least 0 C.

va l ue

2 o

W/m C

relative air humidity" %

In FIG. 3, the different surfaces of the screen are shown completely frosted and all energy supplied is produced in the ice formation layer or passes through said layer from the open air. In this operational condition, there is a low temperature in combination with sufficient humidity. Due to heavy ice formation when the heat pump activates the screen, the ice formation layer at low temperatures of the outside air will still hold 0°C which thus can provide a large energy supply. Under these circumstances it is possible to operate the heat pump at very low temperatures of the outside air, even if the screen primarily operates as an air collector.

In FIG. 4, the screen 18 is shown as a heat exchanger in a heat pump system wherein the screen is located in the open air to recover energy from the surroundings. The heat-carrying medium 20 circulates through the screen, one half of which being connected in series with the other one. The heat pump 19 absorbs energy in the evaporator thereof, and by the compressor the heat energy is raised to a usable temperature level so as to be exchanged by the condenser to the radiator circuit of the object to be heated.

In order that ice formation energy can always be relied upon at cold weather water - moisture - is supplied through a sprinkler system 21 which then sprays water from nozzles 22 over a large surface of the screen. The nozzles which are located at a high level outside the screen or, as in this case, on the screen provides an effective distribution of the moisture due to the prevailing self-convection which is directed downwards.

The sprinkler system is synchronized with the heat-carrier pump of the system in such a way that the screen will be moistened when the rest of the system is activated. Thus, the moistening ceases at shutdown periods and instead the ice crystals will disappear by sublimation - volatilization. The passage of air through the screen is important for an effective sublimation, because the required energy primarily is taken from the air flow. In FIG. 5, there is shown another type of screen having the same function. In this case the screen is located freely and the poles 23 form conduits for the heat-carrying fluid 20. The screens are interconnected in parallel for the lowest possible pressure drop. In FIG. 6, there is shown a detail of the embodiment according to FIG. 5 wherein the screen is fragmentarily shown at 11 at the connection to the conduit 23 for the heat-carrying fluid. The fluid is distributed to the several passages via the manifold 24. The heat-carrying fluid circulated through the wind screen can comprise another fluid than water with anti-freezing agent. E.g. this fluid can comprise freon gas and so-called direct evaporation may be applied.

Claims

CLAIMS 1. Method of recovering ice formation energy wherein a heat-carrying fluid (20) is circulated in heat exchange relationship to an air-permeable screen (18) to cool said screen under ice formation on the surface of the screen due to the humidity of the air, c h a r a c t e r i z e d in that the circulation of the heat-carrying fluid (20) is effected intermittently and is interrupted while the screen is still air-permeable in spite of the ice formation, so as to allow passage of the air through the ice-coated screen (18) in intervals when the circulation is interrupted, in order to accelerate the sublimation of the ice from the surface of the screen, enlarged by the ice formation. 2. Method as claimed in claim 1, c h a r a c t e r i z e d in that moisture for the ice formation on the screen is supplied during the circulation of the heat-carrying fluid (20) by spraying atomized water onto the surface of the screen.