Patent Application
20210388757
The present invention relates to energy storage utilizing air as a storage medium, in order to enable firing high-hydrogen content or 100% hydrogen gaseous fuel in internal combustion engines (ICEs) for electric power generation, with low or zero carbon dioxide emission.
Carbon-free electric power generation utilizing wind and solar resources has dramatically increased their share in the overall power generation portfolio. Advances made in respective technologies have provided opportunities for grid-scale generation at an affordable cost, even without subsidies.
A drawback of either technology is their inability to be available for dispatch when and where needed. The logical solution to this problem is converting the power generated by wind or solar farms at times of low demand into stored energy, which can be readily dispatched when there is demand, but not enough wind or solar power to meet it. One way to accomplish such energy storage is to utilize surplus power generated by wind or solar resources, which otherwise had to be curtailed, i.e., wasted without being put to a good use, to run an electrolyzer to convert water into hydrogen (H2) and oxygen (O2). The hydrogen thus produced can be stored on-site or transported to a different location to be later used as fuel in an internal (or external) combustion engine, e.g., a gas turbine or reciprocating (piston-cylinder) engine, for carbon-free electric power generation. Presently, this method of energy storage has not progressed beyond the demonstration or pilot plant stage. In contrast, a proven method of large-scale energy storage utilizes compressed air as the storage medium.
As shown in
2H2O(l)→2H2(g)+O2(g)
According to the reaction formula, 1/9 (0.11) kg of H2 and 8/9 (0.89) kg O2 is produced from 1 kg of water via electrolysis. Hydrogen generated by the electrolyzer (200) is compressed in an intercooled compressor (202) to the storage pressure. The storage reservoir (203) can be a naturally extant cavern or a man-made vessel. Stored hydrogen can be utilized on-site, in which case the process (204) can include a heat exchanger and a turboexpander or pressure reduction valve to bring H2 pressure and temperature to levels requisite for combustion in an ICE power plant (205), which can be a gas turbine or a reciprocating (piston-cylinder) engine. Depending on the particular combustor technology utilized in the ICE, H2 can be mixed with another gaseous fuel, e.g., methane (CH4). Furthermore, for NOx emissions control, water or steam (H2O) can be injected into the combustor. Obviously, for carbon-free and sustainable power generation, the desirable technology option is 100% H2 combustion without diluent H2O injection.
Another possibility is transporting stored H2 via pipelines and/or tankers, and/or other means to a distant site of utilization, which can be power generation or another industrial facility. In that case, the process (204) includes pipelines with compressor stations, local storage reservoirs, and other equipment to set the pressure and temperature of transported and stored H2 to levels requisite for utilization in an ICE.
One example of proven energy storage technology is compressed air energy storage (CAES), which is depicted schematically in
In CAES technology, surplus power from a carbon-free generation resource such as solar or wind energy (100) is used to drive a compressor (251) to pressurize ambient air to a pressure suitable for long-term storage in a reservoir (252). When there is power demand, stored compressed air is supplied through a valve (253) and recuperating heat exchanger (254) to a gas turbine (255), which burns a fuel to generate electric power. Hot exhaust gas from the gas turbine is utilized in the recuperator (254) to increase the temperature of compressed and stored air before entry into the gas turbine combustor. Since no parasitic compressor power is involved during this process, net power supplied to the grid is roughly twice the value of the same gas turbine with its compressor and burning the same amount of fuel.
Another emerging energy storage technology with air as a storage medium is liquefied air energy storage (LAES). This technology, also referred to as Cryogenic Energy Storage (CES), uses electricity to cool air until it liquefies, stores the liquid air in a tank, brings the liquid air back to a gaseous state (by exposure to ambient air or with waste heat from an industrial process) and uses that gas to turn a turbine and generate electricity. The generic LAES process is shown in
The liquid air is kept stored in a thermally insulated tank (263) (or a multiplicity of tanks) at low pressure. This particular storage technology is cost-effective and mature because similar equipment is widely used for bulk storage of liquid N2, O2 and liquefied natural gas (LNG). Large amounts of energy, of the order of gigawatt-hours, can be stored in commercially available equipment.
During discharge, liquid air is compressed in a cryogenic pump (264) and then heated in a heat exchanger (265). The addition of heat results in air at pressure and temperature suitable for driving a gas expander (266) to drive a synchronous AC generator for electric power generation. This generic (basic) LAES process has low efficiency. The efficiency can be greatly improved by storing the heat extracted during cooling and used during the discharge process for warming the liquid air (not shown in
Hydrogen is a key enabler of carbon-free power generation (here, carbon is a shorthand for carbon dioxide, CO2) because the combustion product is water vapor (H2O). In gas turbines with vintage diffusion combustors, burning 100% H2 gaseous fuel is a mature and proven technology. Nitrogen oxide (NOx) emissions are controlled via diluent water or steam injection into the combustor to reduce flame zone temperature, which is a key driver of NOx formation. The drawback is increasing water scarcity in many geographic locations, which results in strict environmental regulations, limiting the usage of water in power plants. There is also performance degradation associated with irreversible mixing of steam and fuel, lost steam turbine power (if steam is extracted from the bottoming steam cycle in a gas turbine combined cycle) and reduced turbine inlet temperature (for maintaining parts life, because higher H2O content of hot gas increases heat transfer from the gas path to the metal components).
In modern gas turbines equipped with lean premix (known as “dry low NOx” or DLN) combustors, unique properties of H2 such as high flammability, low ignition energy (vis-à-vis methane) and high flame speed present significant design challenges such as flashback, high pressure drop and unstable combustion if H2 content in the fuel gas exceeds 5% (v). All major original equipment manufacturers (OEM) are actively developing DLN combustors that can ultimately handle 100% H2 fuel. (Presently, the state-of-the-art is 30% (v) H2 in the fuel gas).
Similar challenges are present in piston-cylinder engines due to the aforementioned properties of H2, which requires design modifications such as non-platinum tipped spark plugs in car engines (to avoid spark plug electrode temperatures exceeding the auto-ignition limit and causing backfire; platinum can act as a catalyst to H2 oxidation), fuel injectors designed for H2 instead of natural gas in natural gas-fired spark-ignition power-generation units, avoidance of hot spots in the cylinder (to prevent auto-ignition and backfiring) by promoting turbulence and rapid mixing of fuel and air, exhaust gas recirculation (EGR) to minimize NOx production, and others. Other NOx control techniques are equivalence ratios of 0.65 or lower (lean burn) and water injection (same drawback as in the case of gas turbine diffusion combustors, i.e., water scarcity). Major engine OEMs such as Wärtsilä are active in product development to enable higher content of H2 in engine fuel gas (up to 60% (v)). Without development, current capability of gas-fired spark-ignition engines in terms of maximum allowable H2 content in fuel gas is about the same as that in gas turbine DLN combustors, i.e., 5% (v).
Similarly, EGR can also be deployed in gas turbines equipped with diffusion or DLN combustors to suppress the flame zone temperature and reduce NOx emissions. As mentioned above, in diffusion combustors, NOx reduction is achieved via diluent injection (e.g., nitrogen, steam, or water). Utilizing EGR in a diffusion combustor would significantly reduce or eliminate diluent injection, resulting in reduced cost and complexity and increased system thermal efficiency.
The present invention utilizes CAES or LAES in an innovative way to enable up to 100% (v) H2 combustion in a RICE or gas turbine, both internal combustion engines, with high efficiency and low NOx emissions. The present invention discloses a unique way to utilize compressed and stored air in an internal combustion engine (ICE) in an efficient manner, in order to enable combustion of up to 100% H2 fuel in the engine.
The present invention is described in
Note that the surplus power source (100) can be a fossil fuel-fired generation asset as well. One example would be a gas fired combined cycle power plant running overnight (in order not to shut down for fast load ramp up next morning and avoid thermal stresses) and generating power at very low prices (or even a “negative” price, i.e., the storage facility is paid to use the generated surplus power). Another example is a nuclear power plant that must run at full load at all times, whether there is a demand for power or not. This would create an arbitrage opportunity, i.e., the ability to make a profit by discharging stored air and generating power when electricity prices are much higher. The type of the surplus power source is immaterial to the present invention.
During the “discharge” phase of the process, compressed air stream (12) is sent through a pressure control valve (304) to a recuperating heat exchanger (305) and heated to a temperature of 250° C. by cooling the recirculated portion of the RICE (500) exhaust gas stream 24 from the discharge temperature (327° C.) to 70° C. It should be emphasized that the numbers cited herein are meant to be representative of the process based on a particular RICE for illustration purposes and thus subject to change and optimization, particularly by the engine OEM, for actual field deployment. Compressed air stream (14) at 50 bar and 250° C. is expanded through the expander section (601) of a “compander” (combined compressor-expander) (600), which drives the compressor (602) of the same. The compander (600) can be self-balanced (i.e., no net power generation or consumption) as depicted in
Compressor (602), which is on the same shaft as expander (601), increases the pressure of the recirculated portion of the exhaust gas (24) from 1 bar to 5 bar. Recirculated and pressurized exhaust gas stream (25) mixes with the expanded air stream (26) and is cooled in the trim cooler (307) to a temperature of 50° C. Mixed air-gas stream (27) at 5 bar and 50° C. comprises the charge air for the RICE (500), which is ideally designed to burn 100% H2 fuel gas (stream 34).
Exhaust gas stream (21) of the RICE (500) at 1 bar and 325° C. is separated into two streams, 22 and 23. Stream 22 is sent to the stack whereas stream 23 is sent to the recuperator (305). Based on the assumption that recirculated gas stream 23 flow rate is 40% of the RICE exhaust flow stream 21, selected stream properties are summarized in Table 1.
The engine selected for the sample calculation has the following performance: 18.9 MWe generator output; 48% net LHV efficiency, and; 1,180 kg/h H2 consumption (LHV of 120,068 kJ/kg).
The engine is loosely based on MAN 18V51/60 with performance calculated using Thermoflow, Inc.'s THERMOFLEX heat and mass balance simulation software. The base unit is a medium speed (500 rpm), 18-cylinder gas engine available in 18.7 and 20.4 MW electric output versions (MAN 51/60G). Although no detailed information is available on the turbocharger, using typical values of 5 bar charge air pressure and single-stage charge compressor, based on 30 kg/s charge air flow rate, compressor power consumption is estimated as 7.5 MW, which is supplied by the exhaust gas turbine of the turbocharger unit.
According to the present invention, as illustrated in
Alternatively, if the RICE is to be deployed exclusively in an energy storage mode, the turbocharger module can be modified by permanently removing the compressor and adding a generator (
Power consumption of the intercooled compressor (301) during the charge phase of the process is calculated as 11 MW (including the power consumed by the fin-fan coolers for inter- and aftercoolers—not shown in
(1,180/3,600)kg/s×120,068 kJ/kg=39.4 MWth.
Arguably, the most useful measure of CAES performance is the “primary energy efficiency” (PEE), which is the ratio of generated energy to consumed energy, i.e.,
where:
This equation provides an electricity-to-electricity roundtrip efficiency that isolates the energy losses in the conversion of electricity to compressed air and back to electricity. The second term in the denominator of the PEE formula represents the electric energy that could have been generated with the same amount of fuel input with average power plant efficiency (45.5% for natural gas-fired, 32.7% coal-fired power plants in the U.S. in 2015 based on U.S. Energy Information Administration data). Assuming tg=tc and
Thus, the present invention increases the base engine output by 7.5/18.9=40% in energy storage mode at the same fuel consumption as in base operation mode; improves the heat rate (efficiency) by the same factor; and enables (or, at the very least, contributes significantly to enabling) 100% H2 combustion for carbon-free power generation.
It should be noted that the present invention can also be applied to operation with natural gas or a mixture of H2 and methane (CH4), or a syngas (mainly, a mixture of H2 and CO). Exhaust gas recirculation is beneficial to reduction of NOx emissions with any gaseous fuel by the same mechanism, i.e., reduction of flame temperature by dilution with an inert gas component (e.g., H2O and/or CO2). However, unlike the case with combustion of 100% H2, exhaust gas will include CO2.
There are multiple embodiments of the present invention, including: CAES with RICE (simple or combined cycle) and EGR; CAES with gas turbine (simple or combined cycle) and EGR; LAES with RICE (simple or combined cycle) and EGR; and LAES with gas turbine (simple or combined cycle) and EGR.
The invention is described quantitatively using a preferred embodiment, i.e., CAES with RICE (simple cycle) and EGR (
A further embodiment of the present invention, CAES with gas turbine and EGR, is shown in
Another possible admission point for the mixed stream is at the compressor discharge of the gas turbine (510). In that case, stream (27) mixes with the compressed air from the gas turbine compressor discharge and enters the combustor. In order to minimize the temperature discrepancy, recirculated gas and air mixture is heated in the heat exchanger (305) utilizing GT exhaust gas (a portion of it) or working fluid (e.g., steam or water) from the bottoming cycle (700, in the case of a combined cycle configuration). In this case, however, the compander (610) includes the component (603), which is a compressor (i.e., higher pressure ratio and power consumption) instead of a booster “fan.” This is the case because the pressure at the admission point is higher (e.g., 20 bara or more for modern advanced class heavy duty industrial gas turbines) than atmospheric (i.e., 1 bara). In that instance, the likelihood of excess power generation from the compander is unlikely (but not impossible). The particular configuration and performance is subject to optimization during the detailed design phase.
In a further embodiment of the present invention, LAES with gas turbine (in combined cycle) and EGR is shown in
Cryogenically stored air stream (13) from the LAES (250) is heated in the heat exchanger (305) utilizing the exhaust gas (21) from the gas turbine (510). Heated air stream (14) is expanded through the expander section (601) of the compander (610), which drives the booster fan (603) of the same. The compander (610) is designed to generate excess power to be used in a generator. After heating the stored air, gas turbine exhaust gas stream is divided into two branches. Stream (24) goes through the compander (610) and constitutes the circulated portion of the exhaust gas stream (21). Stream (22) goes into the bottoming cycle (700), where it heats the working fluid (steam, hydrocarbon, or another fluid) for additional power generation. The particular configuration is subject to optimization during the detailed design phase.
From the three embodiments shown in
This application claims the benefit of U.S. Provisional Patent Application No. 63/039,048 filed Jun. 15, 2020.
| Number | Date | Country | |
|---|---|---|---|
| 63039048 | Jun 2020 | US |
