Patent Application
20050103033
The present invention generally relates to coolant systems which utilize a pump to circulate coolant through a system and in which system resistance to coolant flow varies over the operating range of the system. More particularly, the invention relates to a pump pressure limiting method which includes reducing the operating speed of the system's coolant pump to maintain coolant pressure within predetermined limits.
Fuel cell technology has been identified as a potential alternative for the traditional internal-combustion engine conventionally used to power automobiles. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%, as compared to a maximum efficiency of about 30% for internal combustion engines. Furthermore, fuel cell power plants produce no hydrocarbon emissions.
Fuel cells, generally, include three components: a cathode, an anode and an electrolyte which is sandwiched between the cathode and the anode. Oxygen from the air is reduced at the cathode and is converted to negatively-charged oxygen ions. These ions travel through the electrolyte to the anode, where they react with a fuel such as hydrogen. The fuel is oxidized by the oxygen ions and releases electrons to an external circuit, thereby producing electricity which drives an electric motor that powers the automobile. The electrons then travel to the cathode, where they release oxygen from air, thus continuing the electricity-generating cycle. Individual fuel cells can be stacked together in series to generate increasingly larger quantities of electricity.
While a promising alternative in automotive technology, fuel cells are characterized by a low operating temperature which presents a significant design challenge. Maintaining the fuel cell stack within the temperature ranges that are required for optimum fuel cell operation depends on a highly-efficient cooling system which is suitable for the purpose.
Cooling systems for both the conventional internal combustion engine and the fuel cell system typically utilize a pump or pumps to circulate a coolant liquid through a network that is disposed in sufficient proximity to the system components to enable thermal exchange between the network and the components. Such cooling systems are usually subject to imposed coolant pressure limits which are based on component or system durability concerns. Because of various factors such as system design constraints and coolant temperature, many of these cooling systems exhibit significant variations or fluctuations in system resistance to coolant flow during the course of normal system operation. Thus, these systems are particularly vulnerable to producing coolant pressures which exceed the coolant pressure limits for the systems. The fuel cell cooling system has been found to manifest a particularly wide variation in coolant pressures over the normal operating range of the system.
Without the use of controls to reduce coolant pressure in a cooling system as needed for maintaining the coolant pressure within the imposed coolant pressure limits, the durability and operational integrity of the system or of system components may be compromised, requiring inordinately frequent system maintenance, repair and/or replacement. Because coolant systems for both fuel cell systems and conventional internal combustion engines lack a mechanism for measuring and controlling coolant system pressures, there is an established need for a method which is effective in controlling the pressures of coolant in a cooling system to prevent coolant pressures from exceeding pressure limitations for the system.
The present invention is generally directed to a novel pump pressure limiting method for preventing coolant in a cooling system from reaching pressures that exceed predetermined system coolant pressure limits. The method includes reducing the system coolant pressure, as needed to prevent system over-pressurization, by reducing the operational speed of a coolant pump used to pump the coolant through the system. In one embodiment, the system coolant pressure is determined directly, by measurement of the pressure of the coolant in the system typically using pressure sensors. The operational speed of the coolant pump is then reduced until the system coolant pressure decreases to within the predetermined pressure limits. In another embodiment, the system pressure is determined indirectly, by obtaining pressure-indicating data such as coolant temperature. The coolant system pressure is then correlated with the coolant temperature or other data and then the operational speed of the coolant pump is reduced accordingly.
The invention will now be described, by way of example, with reference to the accompanying drawing, in which:
The present invention is generally directed to a novel pump pressure limiting method for preventing over-pressurization of coolant in a cooling system in which system resistance to coolant flow, and thus, system coolant pressure, varies across the normal operating range of the system. Such a system is designed to operate within imposed coolant pressure limit specifications beyond which the structural or operational integrity of the system or system components would otherwise be compromised. The system resistance to coolant flow may be a function of system design characteristics, such as structural features, or of properties of the coolant liquid, such as coolant temperature, or a combination of factors. In any case, the operational speed of the pump may be reduced according to the method of the present invention to either decrease the excessive system coolant pressure back to within the predetermined pressure limit specifications for the system or prevent over-pressurization of the coolant beyond the pressure limit specifications. In one embodiment of the invention, a direct measurement of the pressure of the coolant in the system is used to reduce the operational speed of the coolant pump and prevent system over-pressurization when the measured pressure reaches or exceeds the predetermined upper pressure limit. In another embodiment, an indirect measurement of the pressure in the system is made based on parameters such as coolant temperature and pump speed, and the operational speed of the coolant pump is reduced accordingly to prevent system over-pressurization, as needed.
Because coolant pressure in a cooling system usually increases with pump speed, limiting pump speed in potential cooling system over-pressure situations is a viable way to control the coolant pressure in such systems. In cooling systems in which coolant pressures are directly measured by the use of pressure sensors, the pump speed can simply be decreased when the measured coolant pressures reach their limits. However, in many cooling systems, such as automotive cooling systems, coolant pressures are not measured. In these types of systems, other methods are needed to indirectly assess coolant pressures such that coolant pump speeds can be controlled in order to maintain coolant pressures below their upper limits. Cooling systems which exhibit a significant change in system resistance to coolant flow in the normal range of operations are particularly vulnerable to having operating ranges in which pressure limits are exceeded. Fuel cell cooling systems serve as one example of cooling systems the resistance of which changes dramatically over the normal operating range of the system. As a result, fuel cell cooling systems can have relatively large operating ranges in which coolant pressures exceed the pressure limitations of the system.
Referring initially to
In
Referring next to
Referring next to
In some cooling systems, factors such as coolant system loops of various structural characteristics or temperature-independent characteristics of the coolant can induce dramatic variability in the system resistance characteristics. The system resistance characterization in these applications may reveal that pump speeds need to be limited based on a parameter other than temperature, such as another characteristic of the coolant, which particular coolant system loop or flow circuit is in operation, or whether the system is in transition between loop or circuit options. According to the method of the present invention, this system characterization is then used to indirectly determine pressures of the coolant at various pump operational speeds or is experimentally correlated with pressures of the coolant at various pump operational speeds. Based on these coolant pressures, the operational speed of the coolant pump is controlled to avoid operating the cooling system in the over-pressure zone.
Variations in coolant pressure among different ones of multiple coolant system loops in the same cooling system may result from differences in the structural characteristics among the coolant system loops. Other structural characteristics of the cooling system loop, such as variations in the material of construction among different cooling system loops or segments of the cooling system, may contribute to variations in coolant pressures over the operational range of the system. It is also possible that the system resistance characterization will reveal the necessity to limit pump speed based on multiple factors, one or more of which may include temperature and/or structural characteristics of cooling system loops in the system, for example.
Referring next to
While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.
