IL85511A - System for solar energy transfer to a liquid contained in a vessel - Google Patents

Description

3.i ¾ιί η ^na n η>·Λιο ίτλΐίκ imyii nyiyn System for solar energy transfer to a liquid contained in a vessel KERNFORSCHUNGSZENTRUM KARLSRUHE GmbH C: 74067 85511/2 - 1 - System for Solar Energy Transfer to a Liquid Contained in a Vessel The present invention relates to a system for solar energy transfer to a liquid contained in a vessel falling under the generic description of claim 1.

A familiar system of this type comprises directly evaporating collectors and a pot making up the inner part of a double walled vessel, e.g., a cooking pot. The steam hits the vessel on its outer face, the condensate produced is evacuated downward due to gravity. On account of the very good heat transfer during condensation a steam temperature higher by less than 10° will suffice to make boil the water in the pot. However, this familiar system calls for special vessels; conventional cooking pots cannot be used. Moreover, the loop is associated with the danger that uncleaned external faces of the pots will contaminate the collector loop.

A prior ort Japanese Patent No. 60-66059 discloses a solar water heater in which the working fluid. stored in a liquid reservoir can not be sucked up into a return pipe. However, once started, the pressure drop developed at the constriction section of a venturi part allows to suck the gas in the return pipe through a by-pass pipe, resulting in filling the return pipe with liquid by sucking all the gas in the return pipe and as a result generating the syphon effect so as to enable to suck up the working fluid in the liquid reservoir and to supply in to the lower end of an absorber pipe . Accordingly, even in the structure that a flow pipe and the return pipe pierce through a wall located higher than the liquid level in a heat exchange tank, the working fluid can return the absorber pipe without trouble, resulting in bringing the fear of leakage of liquid through the sealing parts between the heat exchange tank. and the flow pipe and the tank and the return pipe to nothing. - la - Starting from this status of the art, the present invention aims at providing a system and a loop, respectively, falling under the generic term of claim 1 which allow to heat liquid in a conventional vessel without requiring external energy in addition to solar energy in order to overcome delivery heights and without necessitating any modifications to be made of the vessel- According to the present invention there is provided a system for solar energy transfer to a liquid contained in a vessel with a) solar collector as a heat source for steam generation by direct evaporation of a heat transfer agent which is conducted in a closed loop; whereby b) the circulation of the heat transfer agent in the loop is carried out by natural convection, as well with c) a heat sink formed as a heat exchanger, provided in the loop, for heat transfer to the liquid contained in the vessel which is placed at a higher geodetical level than the heat source; and d) pipework connecting the inlets and outlets, respectively, of the heat source and the heat sink, whereby; e) the heat sink is formed as a steam condenser which can be immersed into the liquid from the top; f) the steam feed and condensate evacuation lines of the condenser are routed upwards over the edge of the vessel opposite to the force of gravity; g) the condenser consisting of a closed casing; h) the steam line from the solar collector is connected from the top to the inner space of the casing; - lb - i) an immersion tube penetrating the wall of the casing from above whose outlet port at its lower end extends to the lower most point of the inner space; j ) the immersion tube forming one leg of an inverted siphon routed downwards again over the edge of the vessel; characterized in that k) the other siphon leg and the siphon tube, respectively, again routed downwards entering from the top air space of an air collector whose volume constitutes part of the loop volume; 1) the outlet port of said other siphon tube is located in the air collector at a lower geodetic level than that of the immersion tube; and m) the air collector connected with its lower end via a downcomer with the condensate return line leading to the solar collector.

According to the present invention the heating surface in the form of a condenser can be directly dipped into the liquid to be heated and into the charge of the pot, respectively. By _ 2 _ this, direct heat transfer is established which allows to further use in an advantageous manner existing and conventional, respectively, as well as already available vessels. Moreover, the collector loop remains completely closed and has not to be reopened when the heating vessel is withdrawn. This is achieved according to the present invention by placing the heat source directly into the liquid to be iieated so that the external losses can be kept low.

Moreover, with water as the heat transfer agent, no more problems of exclusive heating occur on the heating surface. The heat transfer to the liquid is determinant of the rate of condensation and hence of the heat flow. The maximum temperature achievable can be controlled by the system pressure and thus be kept within limits of about 20°.

It is a particularly advantageous feature of the present invention that the heat transfer agent from the condenser can be lifted over the edge of the vessel for reflow to the collector so that a permanently operating circulating pump can be dispensed with. The loop according to the present invention thus works without energy supplied from outside. This is achieved in the present invention through the interplay of an air volume enclosed in the direction of flow downstream of the condenser and the steam space in the steam line and in the condenser. The condensate is conveyed in this way from the condenser over the edge of the vessel.

Further details of the present invention will be explained in more detail by Figs. 1 to 4.

Figure 1 shows the loop in the cold condition.

Figure 2 shows steady state operation in the loop.

Figure 3 shows the end of the condensate rising phase in the condenser. - 3 - Figure 4 shows the end of the condensate return phase with the water, being the liquid contained in the loop, represented as hatches, the steam as dots and the air with no symbols at all.

Figures 1 to 4 show the solar collector 1 as the heat source which, essentially, consists of the transparent boiling tube 2 as the receiver and the condensate reflow tube 3 in this case placed in a coaxial arrangement in the boiling tube. The steam line 4 runs from the boiling tube upward to the fully closed casing 6 of the condenser 5 as the heat sink which condenser is immersed from top over the edge 8 of a vessel 7 into the liquid 9 contained in it which must be heated. The steam line 4 enters on top the casing 6 at the steam inlet port 28 in order not to interfere with immersion. Two immersion tubes 10 and 11 penetrate through the wall of the casing 6 of the condenser 5 with sealings provided and with the end and outlet port 14, respectively, of the first immersion tube 10 extending to the deepest point 16 of the casing inner space 17, whereas the outlet port 15 of the second immersion tube 11 is placed at a slightly higher geodetic level than the first port.

The two immersion tubes 10 and 11 each form one of the legs of two inverted siphons 12 and 13 whose other downward running legs are termed siphon tube 18 and siphon breaker tube 19. The second siphon 13 is termed siphon breaker tube to describe its function. The two tubes 18 and 19 run downwards geodetically and end in the gas space 20 of a preferably tubular air collector 21 in an almost vertical or inclined position which contains a specified air volume V^. The air collector 21 is connected via a downcomer 22 with the return line 23 through which the condensate and the water, respectively, are returned to the boiling pipe 2 of the solar collector 1 via the condensate return pipe 3. The collector 1, the condenser 5, the - 4 - air collector 21 together "with their connection lines 4, 12, 13, 22 and 23 make up a closed system which is protected against exceeding of the maximum pressure by a safety device not represented in detail here.

As already mentioned, the siphon tube 18 and the other leg, respectively, of the first siphon 12 penetrate into the jas space 20 of the air collector 21 and are Touted downward in it until the outlet port 24 reaches a point below the outlet port 14 of the first immersion tube from the first siphon 12 so that the siphon difference Δ is established. The siphon breaker tube 19 and the other leg, respectively, of the second siphon 13 are connected to the top end 24 of the air collector 21 so that its outlet port 26 ends above the outlet port 15 of the second immersion tube and the siphon difference Δ S2 is established. The siphon breaker tube 19 should be as short as possible whereas the outlet port 26 must be positioned at a geodetic level above the outlet port 15. The first siphon 12 serves to evacuate the condensate 27 from the casing 6, the second siphon 13 serves as an additional pressure equalization through air reflow in non-steady-state operation.

Functioning of the system and of the loop, respectively, will be described below by the four diagrams and process stages, respectively, represented in Figs. 1 through 4.

Figure 1 shows the closed system with the collector 1 and the condenser 5. A filled up vessel 7 acting as the heat sink will surround the condenser 5. In the cold condition the boiling pipe 2 of the collector 1 and the return line 23 are completely filled with water while the riser of the collector 1 and the downcomer 22 are partly water-filled. The rest of loop components inclusive of the condenser 5 contain air at atmospheric pressure. -5 When heat is supplied to the collector 1 and the boiling temperature is attained steam will flow through the riser to the condenser 5. Since the loop is closed, the air originally present in line 4 and in the condenser 5 is displaced by the condenser 5 and the siphons 12 and 13 into the downstream air collector 21 (Fig. 2). The steam condenses in the condenser; the siphon 12 during steady-state operation conveys the condensate 27 to the air collector 22 and to the downcomer 22 and the return line 23, respectively.

However, as the conveyance capacity of the first siphon 12 cannot be adapted to the quick variations of the condensate volumes in the inner space 17 of the condenser 5 which result from the variations in the heat flow withdrawn and, on the other hand, the siphon 12 has to be designed to accommodate the maximum volume, the siphon effect and conveyance would be frequently interrupted. After interruption the condensate accumulates in the inner space 17 and replenishes it from the bottom (see Fig. 3). However, this means a limitation on the heat transfer surface in the condenser 5 on the steam side which implies inadequate condensation for the pressure to rise in the system. Consequently, the air volume available is reduced while the steam generated does not only compensate steam compression but also replenishes the space emptied by reduction in the air volume. For the condensate 27 which, on account of the immersion tube 10 of the siphon 12 pulled down in the condenser 5, acts as the barrier betwe-en the steam and air spaces, this implies a displacement into the air collector 21 so that in the steam line 4 and in the condenser inner space 17 an overpressure builds up which lifts the condensate in the condenser 5 over the siphon 12 by an amount corresponding to the height of the condenser. - 6 - On the other hand, the first siphon 12 interferes with the pressure equalization under conditions of fluctuating heat evacuation, e.g., in case of cold water supply or drop in performance in collector 1 due to the high water column in the siphon tube 18. To avoid resulting fluctuations in the water level going beyond the marked maxima in the collector system, the second siphon and the siphon breaker tube 13, respectively, have been provided in addition- They ensure pressure equalization by reflow of air from the air collector 21 to the condenser 5 while the first siphon 12 can continue the process of conveyance .

The overpressure prevailing in the condenser 5 would cause the accumulated condensate to be transferred also in the absence of the siphon effect, e.g., through the siphon breaker tube 13. However, condensate evacuation into the air collector 21 gives rise to a high rate of condensation in the condenser 5. Making use of a siphon effect also the condensate initially produced can be transferred thanks to the differences in geodetic levels of the ports 14 and 15 so that the frequency of repetition of the filling and evacuation cycles described before is reduced by the response of the first siphon (see Fig. 4).

The ratio of the air volume in the collector 21 to that of the air volume V t in the whole loop should at least attain the same value as the ratio of the atmospheric pressure Patm to the operating pressure p0per- For this to be possible, VA must at least be so large that at a pressure corresponding to the geodetic delivery height the siphon 10 is compressible by the volumes of both siphons 11 and 12.

Another important feature is that a type of sump constitutes the deepest point 16 in the casing 6 of the condenser 5 where the ports 14 and 15 of the immersion tubes 10 and 11 end. This point 16 should be as far distant as possible from the steam inlet port 2J3. ' - ' - 7 - Experiments have confirmed the performance of the loop; the powers conveyed were in the range of 100 ·/· 1500 Watts at a maximum steam temperature of 105°. This corresponds to a saturation range of 1 , 2, 3 bar. - 8 - Nomenclature : 1 solar collector 2 boiling pipe 3 condensate return pipe 4 steam line 5 condenser ¾ casing 7 vessel 8 edge 9 liquid 10 first immersion tube 11 second immersion tube 12 first siphon 13 second siphon and siphon breaker, resp. 14 first outlet port and end, resp. 15 second outlet port and end, resp. 16 deepest point 17 casing inner space 18 siphon tube 19 siphon breaker tube 20 gas space 21 air collector 22 downcomer 23 return line 24 outlet port 25 top end 26 outlet port 27 condensate 28 steam inlet port - 9 - 85511/2

Claims (3)

1. System for solar energy transfer to a liquid contained in a vessel with a) solar collector as a heat source for steam generation by direct evaporation of a heat transfer agent which is conducted in a closed loop; whereby b) the circulation of the heat transfer agent in the loop is carried out by natural convection, as well with c) a heat sink formed as a heat exchanger, provided in the loop, for heat transfer to the liquid contained in the vessel which is placed at a higher geodetical level than the heat source; and d) pipework connecting the inlets and outlets, respectively, of the heat source and the heat sink, whereby; e) the heat sink is formed as a steam condenser which can be immersed into the liquid from the top; f ) the steam feed and condensate evacuation lines of the condenser are routed upwards over the edge of the vessel opposite to the force of gravity; g) the condenser consisting of a closed casing; h) the steam line from the solar collector is connected from the top to the inner space of the casing; i) an immersion tube penetrating the wall of the casing from above whose outlet port at its lower end extends to the lower most point of the inner space; j ) the immersion tube forming one leg of an inverted siphon routed downwards again over the edge of the vessel; - 10 - 85511/2 characterized in that k) the other siphon leg and the siphon tube, respectively, again routed downwards entering from the top air space of an air collector whose volume constitutes part of the loop volume; 1) the outlet port of said other siphon tube is located in the air collector at a lower geodetic level than that of the immersion tube; and m) the air collector connected with its lower end via a downcomer with the condensate return line leading to the solar collector.

2. System according to* claim 1 having the following additional features: n) In addition to one immersion tube, a second immersion tube entering the inner space of the casing of the condenser from the top downwardly, the outlet port at the lower end of this second immersion tube being located at a higher geodetic level than that of the outlet port of the first immersion tube; o) the second immersion tube forming one of a second inverted siphon; p) the other leg again running downwardly as a siphon breaker tube of the second siphon entering the air space of the air collector from the top; q) the point of entrance of the siphon breaker tube into the air collector is located at a higher geodetic level than the point of entrance of the siphon tube from the first siphon into the air collector.

3. System according to claim 2 having the following additional features: - 11 - 85511/2 r) the ratio of air volume VA in the air collector to the total volume of the system tot, in the cold condition being equal to the ratio of the atmospheric pressure Pa-tm to the operating pressure whereby s) VA must be at least so high that under a pressure ratio corresponding to the geodetic delivery height of the firm immersion tube 10, it can be compressed by the volume of both siphon volumes so as to start the siphon effect. DD/mr