Refrigeration – Processes – Compressing – condensing and evaporating
Reexamination Certificate
2000-02-16
2001-08-21
Tapolcai, William E. (Department: 3744)
Refrigeration
Processes
Compressing, condensing and evaporating
C062S160000, C062S196100, C062S228500, C237S00200B
Reexamination Certificate
active
06276148
ABSTRACT:
The air-source heat pump system is the most prevalent type of heat pump used in the world today. This is the case whether one is discussing room units, residential central type, ductless splits, or rooftop commercial systems.
Although the air-source concept in general has a high application potential worldwide, its popularity in the United States and elsewhere has been greatest in mild climate areas. This is because the compressor-derived heating capacity of typical prior art units declines rapidly as the outdoor ambient falls, due, in most part, to the large increase in specific volume (i.e., decrease in density) of the outdoor coil generated refrigerant vapor as the ambient (outdoor) temperature falls. This fall in compressor-derived heating capacity is obviously opposite to the heating requirement, which increases as the outdoor ambient temperature falls. When a typical prior art heat pump operates below its balance point (about 35° F.-40° F.), supplemental heating is required. The most prevalent form of supplemental heat used is electric resistance. In other than mild climates, this use of supplemental electric resistance heat puts the air-source heat pump at a serious economic disadvantage to a consumer as compared with other forms of heating (such as natural gas, oil, and propane), because of the high cost of electric resistance heating.
Electric utilities are also very concerned because of the associated large transformers and distribution systems that are required for any large populations of typical prior art heat pumps whenever high electric resistant (KW) heat backup is required on a regular basis for large population.
When operating at low outdoor ambient temperatures such as 0° F., homes heated by typical prior art heat pumps require as much KW input from the utility as does a home heated by electric resistant KW alone. This is not acceptable to the utility as they would have to increase their generating capacity to supply the demand. In other words, a Northern utility that was summer peaking would now become winter peaking because much more KW output is required for electrically heated Northern homes than what is required for cooling those same homes.
As discussed in my U.S. Pat. No. 5,927,088, one of the areas for capacity and efficiency improvement of air source heat pump systems lies in the recovery of significant heat energy currently remaining in the condensed liquid refrigerant leaving the system condenser. If this remaining energy is recovered and returned directly to the heating side of the system before being thermally degraded and sent to the system evaporator as low density vapor (as is now the case in present day systems), significant increases in compressor derived heating capacity and C.O.P. can be made at lower outdoor ambient temperatures.
The basic problem here is that after the refrigerant has been fully liquefied in the heating condenser, there is still a large amount of energy left in the leaving warm liquid. This remaining energy evaporates a large portion of the leaving liquid itself during the normal pressure reduction process that is required to develop the necessarily low evaporating temperatures. Depending on the refrigerant utilized, and the degree of temperature existing between the evaporator and the condenser, as much as one-half of this liquid can be evaporated during this normal pressure reduction process across the system expansion device when operating at the lower outdoor ambient temperatures.
Obviously, if this liquid has already evaporated, it cannot be again evaporated in the system evaporator, and thus cannot absorb energy from the outside air. However, the net resulting vapor must pass through the system evaporator anyway, creating additional pressure drop along its way, and then must be inducted and fully compressed to the condensing level by the compressor, thus requiring the necessary power to accomplish the compression. The C.O.P. of this portion of the heating process is only one (1) because no energy has been absorbed from the outside air.
Since the compressor must induct this previously evaporated vapor, the compressor can only induct a correspondingly smaller amount of vapor that has been derived from the cooling of outside air (by evaporating the refrigerant liquid that does enter the evaporator along with the previously mentioned vapor). This is not a reasonable process for typical prior art air-source heat pumps operating in other than the milder ambient temperatures because only under those conditions is the relative amount of liquid to vapor (by weight) sufficient to result in a good system C.O.P.
The heating energy output of any heat pump system is also closely proportional to the weight flow of refrigerant vapor entering the system condenser. Approximately 4 times the amount of heat energy is required of 0° F. than is required at 50° F. This means that approximately a 400% increase of entering condenser refrigerant vapor is required at 0° F. ambient as compared to 50° F. ambient in order to adequately match the heating energy requirement. However, the density of the refrigerant vapor generated in the system evaporator when operating at 0° F. ambient is only about 32% of that generated when the outdoor temperature is 50° F. Therefore, when approximately four (4) times the weight flow is required when only 32% of the vapor density is generated, it becomes very obvious that significant changes must be made in order to make an air source pump viable for colder Northern climates.
In addition, if the entire space heating requirement at 0° F. outdoor ambient is to be supplied by compressor derived heating capacity, the air flow across the heating coil of the condenser must be such that the indoor delivered air temperature will be around 105° F. in order to provide adequate freedom from a sensation of cool drafts. This in turn will cause the system condensing temperature to rise to around 115° F. considering a reasonably sized indoor coil surface.
The end result of all this (even if the necessary compressor displacement could be obtained somehow with a present day single compression stage system) is to cause overall system operating compression ratios to rise to the point where it becomes unrealistic to continue the use of typical prior art heat pump technology in normally colder climates. This is exactly what has happened in the marketplace of today. Air source heat pumps are no longer purchased for use in cold climate areas for reasons of both poor comfort as well as the very high cost of the electric energy requirement.
SUMMARY OF THE INVENTION
In this invention, a system and method are presented that achieve a great increase in refrigerant pumping capacity as related to a large fall in the outdoor ambient temperature, combined with a method of extracting energy from warm liquid leaving the system condenser whenever this is needed as well. The heat pump system itself is made to be sufficiently flexible in order to accomplish the necessary goal of both sufficient and efficient heating for a wide range of outdoor ambient temperatures.
In order for air-source heat pumps to become serious contenders for use in colder climates, significant changes must be made for them to realize their true potential. Fundamental Carnot Theory thermodynamic principles unquestionably show that electric powered air source heat pumps indeed do have significant potential in cold climates. In fact, the theoretical Carnot C.O.P. (coefficient of performance) limit for a sink (room) temperature of 70° F. and a source (outside air) temperature of 0° F. is 7.57 units of energy delivered to the sink for every 1 net unit of energy supplied to the compression process.
Carnot C.O.P.=(T
2
&Dgr;S)÷(T
2
−T
1
)&Dgr;S where T
2
is the delivered energy sink temperature (room temperature in absolute degrees) and T
1
is the supplied energy source temperature (outside air temperature in absolute degrees) and &Dgr;S is the constant change in entropy for this theoretical cycle. Therefore, executing the equation, the &Dgr;S's cancel
Cantor & Colburn LLP
Tapolcai William E.
LandOfFree
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