Stoves and furnaces – Solar heat collector – With control means energized in response to actuator...
Patent
1994-01-04
1995-05-09
Yeung, James C.
Stoves and furnaces
Solar heat collector
With control means energized in response to actuator...
126588, 126663, F24J 240
Patent
active
054130913
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
This invention relates to solar energy collectors having means to avoid the bursting of liquid ways in such collectors caused by freezing of liquid contained therein. Without some form of protection, conventional flat plate solar liquid heaters are subject to damage when exposed to temperatures low enough to cause the heat transfer fluid in the collector to freeze solid. Most already adopted methods for overcoming this problem are not fail-safe. Even if the potential of a collector to freeze is only expected as a rare event, collectors require means to either avoid or protect them against the possibility of freezing. Sub-zero ambient temperatures during periods of no solar radiation can readily result in plate and liquid way temperatures below zero. Even where ambient temperatures fall only as low as 5.degree. C., water filled collectors can freeze due to radiant heat loss to a clear night sky. This effect is observed quite commonly in inland areas at times when low humidity in the air leads to high clarity of the sky, hence high radiant heat loss from a collector.
Freeze protection can be provided by draining the liquid from the collectors, use of antifreezing solutions or warming the liquid in the collectors. These known methods work on the principle of avoiding freezing but have disadvantages. For instance, when an antifreeze solution is used in the collector loop it requires a heat exchanger between the collector and the storage tank. Common antifreeze liquids are ethylene glycol-water and propylene glycol-water solutions. Ethylene glycol is toxic as are some commonly included corrosion inhibitors and most plumbing codes require the use of two metal interfaces between the toxic fluid and the potable water supply. This can be accomplished by use of double walled heat exchangers that may take the form of coils in the water storage tank or external to the tank. Either way entails additional cost. Furthermore, glycols decompose over the extended lifetime expected of solar collectors and require replacement, hence further cost.
Alternatively, air can be used as the heat transfer fluid in the collector heat exchanger loop. Whilst no toxic fluids are involved and no double-walled heat exchanger interface is needed, the disadvantage of air heating collectors is their lower effectiveness than liquid heating collectors.
Another method of freeze protection where potable water is used throughout the tank and collector system, is to pump warm water from the storage tank through the collector when necessary to keep it from freezing. This is disadvantaged by thermal losses from the system being significantly increased and an additional control mode must be provided. In emergencies, when pump power is lost, the collector and piping subject to freezing must be drained, requiring a further control means.
A further method adopted is based solely on draining water from the collectors when they are not operating. Since this usually involves draining heated liquid to waste, it detracts from the effectiveness of the collector system as a whole and adds to running costs. Again, a control mode is required which is not fail-safe.
A fifth method is to design the collector plate and piping so that it will withstand freezing. For example, designs have been proposed using butyl rubber risers and headers that can expand if water freezes in them. Another example of such a passive method of coping with freezing and avoiding bursting of conduits is disclosed in lapsed Australian patent application number 75412/81 of C. D. Doughty. The Doughty invention involves the use of differential heat extracting means associated with each conduit, preferably the extracting means being tapered fins or "flanges" as referred to by Doughty. This way freezing is said to occur progressively from one end of each exposed conduit to the other end and expansion due to freezing occurs out of that other end.
The Doughty apparatus and method suffers a penalty in that the fins disclosed associated with riser type conduits are considerably tapered an
REFERENCES:
patent: 4011858 (1977-03-01), Hurkett
patent: 4308856 (1982-01-01), Durand et al.
patent: 4474172 (1984-10-01), Burke
patent: 4611577 (1986-09-01), Clegg
patent: 5143053 (1992-09-01), Baer
Rheem Australia Limited
Yeung James C.
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