Patent Application
20130118170
The present invention relates generally to thermal energy storage systems. More particularly, it relates to systems that store thermal energy making use of a phase change such as liquid water/steam.
There are many circumstances in which it is desirable to store electrical energy in order to resolve temporal mismatches between energy supply and demand. By way of example, the availability of some alternative energy sources such as solar energy and wind energy can vary significantly over the course of a day and the output of devices that harvest such energy, (e.g., photovoltaic collectors or wind turbines) is often mismatched with the demand for the electrical energy that such devices produce. Another example of the mismatch between supply and demand is embodied in baseline electricity generating facilities such as nuclear powerplants which are generally designed to provide a substantially steady electrical output while the demand for electricity tends to vary significantly over time.
The mismatch between supply and demand can make bulk electrical energy storage desirable and over the years, a wide variety of systems and devices have been proposed and/or used to facilitate such storage and retrieval electrical energy. One of the most familiar examples of a device designed to store electrical energy is an electrical battery. Although batteries tend to work well for relatively small-scale energy storage applications, they tend to be cost prohibitive in larger scale energy storage systems. An example of a larger scale energy storage system that has been used in grid scale electrical energy storage applications is pumped hydro-electric power and storage. In a pumped hydro-electric power storage system, power is generated by a turbine which extracts the potential energy of water flowing from an upper reservoir to a lower reservoir. When demand for power is low, and there is an excess of supply from other power sources at low cost, the turbine can be reversed to pump water from the lower reservoir to the upper reservoir, thereby storing energy as gravitational potential energy. At a later time, when the demand for power is higher, the water stored in the upper reservoir can be used to drive the turbine to generate electrical power.
Other medium and large scale energy storage systems have utilized mechanisms such as thermal energy, compressed air, flywheels, electrical capacitors and chemical energy as the energy storage mechanisms. Although such conventional energy storage systems have a number of benefits, there are continuing efforts to develop cost effective energy storage systems having relatively high round trip energy recovery efficiencies. Such devices can make it practical to acquire or purchase electricity and store the associated energy when energy availability is higher than demand and its price and/or value is low, and to retrieve and utilize or sell such energy when energy availability is less than demand and its price and/or value is high.
A variety of energy storage and retrieval systems are described. Generally “hot” and “cold thermal reservoirs are provided. The “hot” thermal reservoir is arranged to hold a working fluid in both a liquid phase and a saturated vapor phase state. The “cold” thermal reservoir is arranged to hold the working fluid in a second state having a lower temperature than the working fluid in the hot thermal reservoir. A heat engine/heat pump unit is arranged to: (a) extract energy from working fluid passing from the hot reservoir to the cold reservoir via expansion of the working fluid in a manner that generates mechanical energy to facilitate retrieval of energy from the energy storage and retrieval system; and (b) compress working fluid passing from the cold thermal reservoir to the hot thermal reservoir to facilitate the storage of energy in the energy storage and retrieval system.
A variety of different materials can be used as the working fluid. In some preferred embodiments, water is used as the working fluid. When water is used as the working fluid, it passes through the heat engine/heat pump unit as steam. In such embodiments, the heat engine/heat pump unit may take the form of a steam engine.
The heat engine/heat pump may take the form of a reversible heat engine that can act as both an expander and a compressor or separate devices may be used as the expander and compressor. In some embodiments, a reversible positive displacement heat engine (e.g. a piston steam engine) is used as the heat engine/heat pump. By way of example, unaflow, universal unaflow and counter-flow steam engines work well. In some preferred implementations, the heat engine includes adjustable valves and a controller arranged to vary the timing of the opening and closing of the valves relative to a crankshaft angle.
In order to facilitate the storage and retrieval of electrical energy, an electric motor/generator unit may be mechanically coupled to the heat engine/heat pump unit. The electric motor/generator is arranged to drive the heat engine/heat pump unit during the compression of working fluid and to generate electricity during the expansion of the working fluid. The electric motor/generator may be implemented as a single reversible unit or as separate motor and generator devices.
The hot and cold reservoirs may take the form of pressure vessels or unpressurized reservoirs. When an unpressurized reservoir is used, a sub-atmospheric pressure chamber may be provided to facilitate sub-atmospheric flashing of working fluid to vapor (e.g., liquid water to steam) and/or sub-atmospheric condensation of vapor to a liquid. When a pressurized vessel is used as the hot reservoir, the working fluid in the hot reservoir may be stored at a pressure substantially above ambient atmospheric pressure.
In some alternative embodiments, the cold store may be replaced by the combination of a condenser arranged to condense steam and a separate low temperature thermal energy source arranged to provide steam having a lower temperature than the hot reservoir.
The invention and the advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:
a) is a schematic diagram of an energy storage and retrieval system having a pressurized hot reservoir and an unpressurized cold reservior with a sub-atmospheric pressure flashing chamber operating in an energy storage mode.
b) is a schematic diagram of the energy storage system of
In the drawings, like reference numerals are sometimes used to designate like structural elements. It should also be appreciated that the depictions in the figures are diagrammatic and not to scale.
A number of thermal energy based energy storage and retrieval systems are described. In general, the described storage systems utilize a working fluid having a substantial heat capacity and make use of a phase change to help improve the storage capacity of the system (e.g. a liquid water/steam phase change).
Referring next to
When operating system 100 in a manner that retrieves energy, the heat engine/heat pump 120 is operated in a heat engine mode. The heat engine 120 drives generator 130 which in turn generates electricity which can be used to power an electrical load, delivered to the power grid, or utilized in any other desired manner. Although a system that receives and delivers electrical energy is illustrated, it should be appreciated that in alternative embodiments, mechanical energy (such as a rotating shaft driven by a wind turbine or other device) could drive the heat pump and/or the mechanical output of the heat engine could be used directly as desired.
The hot thermal reservoir 105 holds working fluid at a higher temperature than the cold thermal reservoir 110 although the actual temperatures of the reservoirs may vary widely. Thus, as used herein, the labels “hot” and “cold” are merely intended to indicate the relative temperatures of the reservoirs as opposed to the specific temperatures of the working fluid within the reservoirs or their temperatures relative to ambient temperatures or any specific reference temperature. As will be described in more detail below, the thermal reservoirs 105 and 110 may take a wide variety of forms and may store either pressurized or unpressurized working fluid.
In the described embodiments, steam is used as the working fluid within the heat engine/heat pump 120 and therefore water/steam is stored in the thermal reservoirs 105 and 110. However, it should be appreciated that in alternative embodiments, a wide variety of other working fluids could be used in place of water. By way of example, various water based mixtures (e.g., water/ammonia mixtures), fluorocarbons, ammonia, hydrocarbons and other refrigerants and/or mixtures that include any of these fluids can be used in alternative embodiments. In general, it is desirable to store the working fluid primarily in a liquid phase so that a phase change can be used advantageously to improve the storage capacity of the system.
To facilitate a working explanation of the energy storage and retrieval system 100,
Since the hot store 105 is pressurized, there will inherently be both saturated steam and pressurized water within the hot store. Although there may be some temperature stratification within the reservoir, there will generally be a thermodynamic equilibrium between the saturated steam and the liquid water at the liquid/vapor boundary (i.e., the water surface). As compressed steam is introduced to the hot store, both the temperature and the pressure of the hot store will increase while generally maintaining a thermodynamic equilibrium at the liquid/vapor boundary. To help reduce temperature stratification within the hot store, it is often desirable to inject the steam near the bottom of the hot store as illustrated in
Since the hot thermal reservoir is effectively a closed system, the actual operating pressure of the hot store at any time will be highly dependent on its current temperature (i.e., higher temperatures=higher pressures). As more compressed steam is added to the hot store, both its temperature and pressure will continue to rise. However, it is desirable to set some limit as to the maximum operational temperature/pressure for the hot store. In practice, the maximum operational temperatures/pressures tend to be dictated by economic constraints, with a primary constraint being the cost of the vessel(s) used as the hot thermal reservoir. Currently, there are a number of pressure vessels (tanks) commercially available that are rated to pressures of about 17 or 18 bar gauge (i.e., about 17 or 18 atmospheres). At a pressure of 17 bar gauge (about 250 psig) the temperature of the water and steam within the hot store would be about 207° C. Since such pressure vessels are commercially available at relatively low costs, it may be desirable to design a system such that the maximum operational pressure of the hot store 105 is in that range. However, that is by no means a requirement and the maximum operational pressure of the hot store may be widely varied to meet the needs of any particular application.
As indicated above, the cold store 110 serves as a source of steam for the heat pump 120(a). In the embodiment of
In the embodiment illustrated in
In the energy storage mode, steam is drawn from the flashing chamber 113 and compressed by the heat pump 120(b) as illustrated in
When a flashing column is used, the actual design of the column may be widely varied in order to meet the needs of any application. Typically, it will be important to provide sufficient water surface area within the flashing chamber(s) to ensure that steam can be generated at a rate high enough to supply the heat pump 120(b) at the desired flow rates. In some applications it will also be desirable to provide a mechanism for circulating water around the cold thermal reservoir 110 and the flashing column 112. This mixing of the water helps reduce the risk of temperature stratification within the flashing column 112. Specifically, it should be appreciated that when a significant amount of steam is being generated, the water at the flashing surface will cool relatively quickly and thermosiphoning alone may not be sufficient to maintain the temperature of the water at the flashing surface at close to the same temperature as the main body of water within the cold store 110—especially if the diameter or width of the column (and thus the flashing surface area) is small relative to the column height. It is generally undesirable for water at the surface of the flashing chamber to be at a temperature that is significantly below the temperature of underlying body of water since that would cause a reduction in the temperature and pressure of the steam being supplied to the heat pump 120(b) which would reduce the overall efficiency of the system.
A variety of mechanism can be used to enhance circulate of the water within the cold store and flashing chamber. For example, impellers, propellers and other mixing devices can be appropriately positioned within the cold store (e.g. in the column 112) and used to enhance circulation and mixing. In various other embodiments, two or more columns may be used to enhance circulation.
Referring next to
When steam is drawn from the hot store 105, the pressure within the store will decrease, which in turn causes more water to vaporize (flash) to bring the hot store 105 back towards equilibrium. Such vaporization of some of the liquid water into steam extracts heat from the surrounding water thereby effectively cooling the surrounding water. Therefore, as steam is drawn from the hot store 105, the temperature and pressure of the water/steam within the hot store will gradually drop.
As will be appreciated by those familiar with heat engines, the adiabatic expansion of a gas will cause the temperature of the gas to drop significantly during the expansion process. Thus, the expanded steam exhausted by the steam engine 120(b) will be substantially cooler than the input steam and may be returned to the cold store 110. In order for the steam engine to get the most work out of the steam (and therefore to operate most efficiently), it is generally desirable to exhaust the steam at as low of a pressure as possible. As such, it is often desirable to exhaust steam from the steam engine at a pressure that is below atmospheric pressure. Since the pressure within the flashing chamber 113 is below atmospheric pressure, the flashing chamber serves as a good location to receive the exhaust from the steam engine.
Conceptually, when steam is delivered to the flashing chamber 113, the pressure within the flashing chamber increases, which causes some of the steam within chamber 113 to condense to bring the chamber 113 back into equilibrium. Thus, in the energy retrieval mode, flashing chamber 113 effectively acts as a sub-atmospheric condensation chamber. The condensation of some of the steam back into liquid water adds heat to the cold store thereby effectively warming the surrounding water. Therefore, as steam is introduced to the flashing chamber 113, the temperature of the water within the cold thermal reservoir 110 will gradually rise.
As steam is injected into the cold store, precautions are preferably taken to help insure efficient condensation of the steam and good thermal mixing of the condensate with the water in the cold store. One way to enhance condensation within the chamber 113 is to spray water droplets into the chamber as steam is introduced to the chamber. The water droplets enhance condensation of the steam since steams tends to condense on the droplets. Such spraying can be accomplished by a sprayer 114 which draws water from the cold store and sprays a shower of water from the top (or near the top) of the column, thereby effectively creating a shower of water within the condensation chamber 113. A simple pump (not shown) may be used to draw water from any suitable location in the cold store and spray it into the condensation chamber to thereby enhance condensation. Other conventional condensation enhancing mechanisms such as drip trays and Raschig rings, trays or other structures arranged to enhance the exposed surface area of water (not shown) can be used as well.
It is also desirable to insure that there is good mixing between water at the surface of the flashing chamber and the underlying water in the cold store. Without good mixing, the temperature at the surface will rise relative to the underlying pool as the energy from the condensing steam heats the water near the surface. Higher temperatures at the water/vapor boundary have the effect of increasing the pressure within chamber 113. The increased pressure within chamber 113 means the steam can be expanded less in the steam engine, which reduces the work that can be extracted from the steam, thereby reducing the overall Round Trip Efficiency of the energy storage and retrieval process. The same mixing mechanisms used to enhance circulate of the water within the cold store during steam generation can also be used to circulate water during condensation.
The described energy storage and retrieval system 100 is well suited to facilitate thermal storage of energy. When excess electrical or mechanical energy is available, such energy can be used to drive the heat pump 120(a) in a manner that compresses and heats working fluid (e.g. steam) drawn from the cold thermal reservoir 110 for storage in the hot thermal reservoir 105. When it is desirable to retrieve energy from the storage system, hot, high pressure steam is drawn from the hot store and expanded in heat engine 120(b) to extract useful work from the steam that can be used to generate electricity or for any other desired purposes. In an ideal system, in which the heat engine/heat pump 120 and the motor/generator 130 do not have any losses and there are no thermal or pressure losses (or gains) in the thermal reservoirs, the Round Trip Efficiency of the energy storage can be 100%. That is, the amount of useful energy that could be retrieved from the system would theoretically be the same as the amount of energy used to drive the system. Of course, a 100% round trip storage/retrieval efficiency (RTE) is not possible in a practical system, however even when the electricity is used to power the system and electricity is the form of the power ultimately output by the system, round trip storage/retrieval efficiencies on the order of 60-80% are believed to be readily obtainable using existing technology (e.g., using a positive displacement steam engine/compressor in conjunction with a motor/generator as described in more detail below) and even higher round trip efficiencies may be possible. Higher efficiencies are also possible in systems that would directly supply and/or utilize the mechanical power that drives and/or is output by the heat engine/heat pump since any inefficiencies of the motor/generator 130 would be eliminated. As will be apparent to those familiar with grid scale energy storage applications, there are a number of applications where round trip storage and retrieval efficiencies in the 60-80% range are economically viable.
In the embodiment illustrated in
An advantage of the type of system illustrated in
Yet another embodiment is illustrated in
From the foregoing, it should be apparent that the size, geometry and layout of the thermal reservoirs 105 and 110 may be widely varied within the scope of the invention. In practice, many tanks and vessels have a circular cross section although this is not required. In a pressurized hot stores, it is often preferable to orient the tank(s) in a generally horizontal manner as illustrated in
When a sealed tank is used as the cold store, the tank should be capable of withstanding the vacuum pressure of the flashing chamber (e.g., up to 1 atmosphere of negative pressure). In some applications, the use of such vacuum pressure resistant tanks may be more cost effective than providing a non-pressurized tank with a flashing column 112. By way of example cylindrical tanks having a height of at least 10 meters (as for example 12 meters to match shipping container length) work well for many applications.
In most of the examples given above, the thermal reservoirs are described primarily in the context of various tanks and pressure vessels. Although tanks and pressure vessels work quite well, it should be appreciated that a wide variety of different structures can be used as the thermal reservoirs when appropriate. By way of example, it available, a lake, pond or other defined body of water could be used as an unpressurized thermal reservoir—and particularly as the cold store. When readily available, the use of such bodies of water may have several potential advantageous. For example, if the total mass of water in a lake or pond it very substantially more than is used by the energy storage system, then the overall temperature of a lake used as a cold reservoir may not fluctuate significantly through the course of an energy storage and retrieval cycle. A cold store that maintains a relatively constant temperature tends to facilitate more efficient storage. In other embodiments cisterns and various in-ground containments can be used as one or both of the thermal reservoirs.
In most of the illustrated embodiments each of the thermal reservoirs takes the form of a single containment. However it should be appreciated that either or both of the reservoirs may be formed from any number of individual containment structures. Indeed, in some applications, modular containments may be preferable. For example, in one specific application a number of modular tanks each sized to fit within a shipping container may be used to form one of the thermal reservoirs (e.g., a cold store).
Typically it is desirable to insulate the hot store in order to avoid thermal losses in the primary energy store. Depending on the location and operating conditions, it may, or may not, be desirable to insulate the cold store. The reason that insulation may be less desirable on the cold store is that inefficiencies in the storage and retrieval process will typically add heat to the system and such heat tends to migrate to the low temperature store. More specifically, during a charge/discharge cycle, a portion of the energy lost due to inefficiencies of the compressor and heat engine will appear as a net heat flow into the cold store. This amount of heat typically needs to be removed from the system to prevent the temperature of the cold store from rising to an inefficient level over time. A variety of conventional mechanisms may be used to regulate the temperature of the cold store 110. As such, some natural cooling of the cold store may be desirable to help maintain system balance.
Referring next to
In the illustrated configuration, the hot store 105(c) is an insulated pressure vessel containing water and a saturated steam space that operates as described above. The low temperature energy source may take the form of an unpressurized water vessel heated by any suitable heat source. The low temperature sink 175 is a condenser cooled by an available heat sink The heat sink may take any suitable form and the most appropriate heat sink may vary by location. By way of example, a stream or other body of water, evaporative coolers, fin-fan coolers or a variety of other heat exchangers may be used to cool the condenser.
In general, the low temperature energy source 185 is maintained at a temperature that is higher than the sink 175. When storing energy, water in the low temperature energy source 185 is flashed into steam as previously discussed with respect to the cold store 110. As before, a column or other suitable vessel structure may be used to facilitate sub-atmospheric flashing of the steam. The temperature of the water and thus the saturated steam supplied by the source 185 is generally higher than it would be if a cold store was used. Since the temperature of the water is higher, the pressure of the steam input to the heat pump 120(a) is higher which means that less energy is required to compress the steam for storage in the hot store 105.
When retrieving energy, the steam engine 120(b) exhausts steam into the low temperature sink 175 (as opposed to the source 185). The sink 175 condenses the steam and is cooled by the available heat sink Like the cold store, the sink 175 includes a sub-atmospheric condenser 177 that facilitates condensation of the exhaust steam at sub-atmospheric pressures. In general, the temperature of the sink 175 is cooler than the temperature of the source 185. Since the temperature of water within the sink's condensation chamber is lower than the temperature of the source, the pressure within the condenser 177 (and thus the pressure at which condensation occurs) is lower as well. Therefore, steam can be exhausted from the steam engine 120(b) at a lower pressure than it could if the steam was returned to source 185. This allows the steam engine to extract more energy from the expansion of the steam, thereby improving the efficiency of the storage and retrieval system.
It should be appreciated that in an ideal system operating with the arrangement of
Optionally, condensate from the condenser in sink 175 can be recirculated to low temperature heat source 185 where it is heated before being used to generate steam. An advantage of such recirculation is that it potentially reduces the system's overall consumption of water.
It should be apparent that there are a number of industrial and power generation processes that generate waste heat which could be utilized in the type of system illustrated in
The size of the reservoirs that are suitable for use as the hot and cold thermal reservoirs 105, 110 can vary with the desired energy storage capacity of the system, the pressure ratings of the vessels, tanks or other structures used as the reservoirs and/or flashing columns, the operating temperatures ranges (which may be dictated in part by the permissible operating pressure ranges), etc. By way of example, to give a rough scale of a suitable system size, a system designed to deliver 1 Megawatt (MW) of power for 10 hours (and thus has over 10 MW hours of storage capacity) could be implemented using a 170,000 gallon hot store that can be pressurized to about 17 bar (about 250 psi) and a 500,000 gallon unpressurized cold store. In this particular example, the cold store has about 3 times the volume of the cold store. However, it should be appreciated that the actual relative volumes of the hot and cold store may vary widely and that the relative prices of the vessels that are available for use as the thermal reservoirs may strongly influence the ultimate reservoir sizes selected for any particular power storage capacity.
It should be appreciated that the size of the hot store can generally be reduced if the vessel used as the hot store can withstand higher operating temperatures and pressures. Conversely, the size of the hot store will have to increase if the vessel used can only withstand lower operating temperatures and pressures or if the hot store is unpressurized. Furthermore, it should be appreciated that the volume of the cold reservoir can significantly affect the required volume of the hot reservoir as well. More specifically, as indicated above, during operation in the energy storage mode, thermal energy is withdrawn from the cold store 110 thereby cooling the cold store and thermal energy is added to the hot store 105 thereby heating the hot store. Thus, the temperatures of the thermal reservoirs will diverge during energy storage. Conversely, during energy retrieval, thermal energy is withdrawn from the hot store 105 thereby cooling the hot store and thermal energy is added to the cold store 110 thereby heating the cold store. Thus the temperatures of the thermal reservoirs will converge during energy retrieval. As the temperatures converge, the amount of energy that can be retrieved from steam drawn from the hot store will decrease.
It should be appreciated that the temperature of a cold store that is the same size as the hot store will vary significantly more than the temperature of a cold store that has three times (3×) the volume of the hot store. Thus, given identical hot stores a system that has a 3× cold store will have the ability to extract more energy than a system with a cold store that is the same size as the hot store. By extension, if a still larger cold store is use (e.g. a 10× cold store) it is possible to extract even more energy from a hot store of the same size. If a relatively large body of water such as a lake that has many, many times the volume of the hot store, then the cold store may stay substantially the same temperature throughout an energy storage and retrieval cycle. Such a system would have the ability to extract even more energy given an identical hot store which provides a larger usable energy storage capacity. Thus, it should be apparent that selection of the relative sizes of the hot and cold reservoirs will often be based primarily on the cost benefit analysis given available resources, tanks and pressure vessels.
Either of the hot and cold thermal reservoirs may be implemented as single tank, vessel or reservoir, or may be implemented as multiple tanks, vessels and/or reservoirs. By way of example, a 500,000 gallon hot store could be implemented as seventeen 30,000 gallon pressure vessels which might each be about 40 feet long and 12 feet in diameter.
The heat engine/heat pump 120 can be implemented in a variety of different manners. Although a variety of heat pumps and heat engines including steam turbines may be used, one class of heat engines that is particularly well suited for use as the heat engine/heat pump 120 are positive displacement piston steam engines. One advantage that piston steam engines have over turbines and other conventional heat pumps and heat engines is their relatively high operating efficiencies over a wide range of inlet and exhaust pressures and temperatures. This is particularly useful in the described energy storage and retrieval systems because it allows the temperatures and pressures of both the hot and cold thermal reservoirs to vary significantly over the course of an energy storage and retrieval cycle without drastically reducing the system's round trip energy storage and retrieval efficiency.
Another advantage of piston steam engines is that they can operate relatively efficiently over a range of steam mass flow rates which allows the system to efficiently draw or deliver energy even in the face of variable energy supply and demand. This capacity meshes well in applications where the energy available for storage at any given time may vary significantly during the course of a day, such as is inherent in solar and wind farms. It also fits extremely well in applications that have variable demand for energy during retrieval (which are many).
Many piston steam engines are reversible in that they may be operated as both as a heat engine (expander) and as a heat pump (compressor). This can be advantageous because it allows a single machine to be used as both the heat pump and the heat engine. Although a reversible device is useful in many applications, it is not required. Rather, when desired, separate devices can be used as the heat pump and the heat engine and the heat pumps/heat engines shown in the drawings are intended to represent both approaches. This may be desirable, for example, when separate expanders and compressors are less expensive and/or more efficient than a reversible device. Furthermore, it should be appreciated that more than one steam engine (or other devices) may be used in parallel as the heat engine/heat pump in order to deliver the desired throughput.
It should also be appreciated that despite the fact that positive displacement piston steam engines are not currently popular, piston steam engines are a very mature technology. Indeed, piston steam engines were the dominant source of power well into the 20th century. Accordingly, there are a number of existing piston steam engine designs that may be used as the heat engine/heat pump. These include single stage steam engines and multi-stage (multiple expansion stages) steam engines, etc. In some specific applications, a single or multi-stage Unaflow, Universal-Unaflow or counter-flow piston steam engine may be used. One such engine has been developed by the Applicant and is based on the Skinner Universal-Unaflow steam engine from the 1930s.
The actual number of serial stages that are desirable and appropriate for any particular application will vary in accordance with a number of factors including the expected temperature differential between the thermal reservoirs, the availability of steam separators and water injectors, costs and other factors. In some applications, a single stage of expansion may be preferred whereas in other multiple stages (typically in the range of 2-5 serial stages) would be preferred.
Unaflow steam engines and Universal-Unaflow steam engines are piston based steam engines that utilize poppet valves to control the introduction of steam into the cylinder that serves as an expansion chamber. Conventionally, the timing of the valves has been controlled by a camshaft. However, it should be appreciated that other known valve timing control mechanisms can be used as well. For example, electronic solenoids may be used to open and close the valves which facilitates electronic control the timing of the opening and closing of the valves if desired. Regardless of the mechanism used, there are several advantages to providing a wide range of control over the relative timing of the opening and closing of the valves. For example, in order to maximize efficiency, it is generally preferable for the compression ratio of the steam engine/heat pump to relatively closely match the pressure ratio between the pressures of the hot and cold stores. Varying the timing of the opening and closing of the intake and exhaust valves is one good way to accurately control the compression ratio of the engine. Varying the intake valve cutoff timing also provides a good mechanism for controlling the mass flow rate of steam through the steam engine which allows good control of the power generated by the steam engine during expansion.
Adjustable valve timing can also be very useful in facilitating reversible operation of the steam engine such that the steam engine may be operated as either an expander (a traditional steam engine) or as a compressor (heat pump). Conceptually, many steam engines, including piston and other positive displacement steam engines may be operated as either an expander or a compressor. The primary difference is that in the expander mode, high pressure steam is drawn into the expansion chamber and is used to drive a piston that delivers useful work to a crankshaft, whereas in the compressor mode, an electric motor (or other suitable power source) powers the crankshaft which drives the pistons to compress steam within the chamber (which now acts as a compression chamber instead of an expansion chamber). If the timing of the opening and closing of the valves remains the same (relative to crank angle), then to change from expansion to compression, the rotational direction of the crankshaft must be reversed. This requires stopping the steam engine and reversing its direction. This works fine in energy storage systems where rapid shifts between power consumption and power generation are not required—such as when storage is expected to occur primarily at night when electricity rates are low and retrieval is expected to occur primarily during other parts of the day when electricity rates are higher. However, there are also a number of applications, such as load balancing, where the ability to quickly shift between power storage and power delivery modes is quite valuable (i.e., shifting modes in seconds rather than minutes). In a piston based steam engine, this type of rapid shifting between power generation and power storage modes is possible if the crankshaft can continue to rotate in the same direction when shifting modes. This can be accomplished by altering the relative timing of the opening and closing of the valves—whose functions (intake and exhaust) are, of course, reversed when the operational mode is reversed.
As is well known, anything close to the adiabatic compression of steam will cause the superheating of the steam. Too much superheating of the steam may be undesirable since it can lead to thermal losses in the system. Therefore, in some embodiments, one or more water injectors are provided to spray water into the steam before or during compression so that some of the thermal energy generated during compression is absorbed by vaporization of the injected water. The injector(s) may be arranged to inject water directly into each cylinder during compression or they may be arranged to inject the water into the steam prior to its introduction into a compression cylinder. In general it is desirable to keep the compressed steam that exits the steam engine during compression at a state that is close to saturation so that excess superheating does not occur. However, that is not a requirement. It should be appreciated that the desired amount of water to inject will vary as a function of several different factors such as the compression ratio currently in use, the mass flow rate of steam passing through the cylinder(s), the entrance temperature and pressure, etc. Therefore, it is preferable to control the volume of water injected at any given time based on the operating state of the engine so that a point close to saturation can be achieved in the compressed steam exiting the steam engine. If more than one compression stage is used, then water injection may be desired for each compression stage, although again, this is not a requirement.
Conversely, adiabatic expansion of saturated steam can cause some of the steam to condense during the expansion. It is generally undesirable to introduce wet steam (i.e., steam that includes water droplets) into a cylinder because the condensation can erode inlet valve ports. Therefore, in multi-stage steam engines, it is often desirable to pass the steam through a steam separator (not shown) between sequential stages in order to remove most of the water droplets from the steam. It may also be desirable to pass the steam through a steam separator before it enters the first (or only) expansion chamber (cylinder). Any water condensed by the steam separators can be returned to one of the thermal reservoirs (typically the cold reservoir) to help capture the energy contained therein. A variety of different conventional steam separators can be used to remove water droplets from the steam. By way of example, mesh steam separators work well.
In other embodiment it may be desirable to superheat the steam before it enters the steam engine and/or to reheat the steam between sequential stages. This can be accomplished through the use of an optional superheater that heats steam drawn from the hot store before it enters the steam engine, and/or an optional reheater that heats steam between sequential stages of the steam engine. Although the use of superheaters and/or reheaters is possible, they are often not cost effective unless the heat source(s) used to power the superheaters and/or reheaters are waste heat sources or other very low cost energy sources.
It should be apparent that the various described energy storage systems are well suited for the delivering low cost bulk energy storage and retrieval with a high Round Trip Efficiency. Storage and retrieval intervals can be as short as a few seconds and as long as hours or days. Intermittent power sources such as wind and solar tend to exacerbate the mismatch between energy supply and demand, thus increasing the need for such energy storage and retrieval systems.
Although only a few embodiments have been described in detail, it should be appreciated that the invention may be implemented in many other forms without departing from the spirit or scope of the invention. For example, the specific construction and layout of the hot and cold thermal reservoirs and the specific devices used as the heat pumps and heat engine may be widely varied. The hot and cold stores may each be implemented as a single tank or as multiple separate tanks. When multiple tanks are used for one of the stores, the working fluid within the different tanks in the same store may be a substantially the same temperature or at different temperatures. As previously mentioned, one or more reversible heat engine(s)/heat pump(s) may be used as the expander/compressor or separate devices may be used for expansion and compression. Similarly, in electricity storage and retrieval applications, a reversible motor/generator may be used to drive/be driven by the heat engine/heat pump, or separate motor(s) and generator(s) may be used. The described systems are readily scalable and may be used in a variety of different energy storage applications, including grid scale energy storage and retrieval applications.
It should also be appreciated that if sources of low cost heat are readily available, such heat can be used to help improve the efficiency of the system. One such arrangement was illustrated in
Effective Round Trip Efficiencies of at least 60, 70 and even 80% can be attained using the described approach. Although higher Round Trip Efficiencies are generally desirable, cost considerations may dictate the system design and thus the attainable Round Trip Efficiency. However, systems having 60-80% (or higher) Round Trip Efficiencies are believed to be economically viable in a number of specific applications.
The described embodiments should be considered illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.
This application claims priority to U.S. Provisional Patent Application No. 61/559,318, filed Nov. 14, 2011, which is hereby incorporated by reference in its entirety for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 61559318 | Nov 2011 | US |
