WO2023244465A1

WO2023244465A1 - Utility-scale underground hot water storage (usuhws) for power production and heat supply

Info

Publication number: WO2023244465A1
Application number: PCT/US2023/024560
Authority: WO
Inventors: Yiding Cao
Original assignee: Individual
Current assignee: Individual
Priority date: 2022-06-18
Filing date: 2023-06-06
Publication date: 2023-12-21
Anticipated expiration: 2024-12-18

Abstract

This invention provides constructed utility-scale underground hot water storage (USUHWS) systems to enable renewable energy sources to produce power or supply heat reliably and continuously with minimized interruption and impact by weather conditions. In combination. with renewable-energy-based thermal power plants, said USUHWS systems could have the potential to phase out most fossil fuels for power production and heat uses. The USUHWS systems could retain the thermal energy content of the stored hot water for a long time so that renewable energy can be extracted and stored at any time and in any season and be used whenever needed.

Description

Utility-Scale Underground Hot Water Storage (USUHWS) for Power Production and Heat Supply

This application is a continuation in part of a US provisional patent application 63/353,581 filed on 18- JUN-2022.

Field of Invention

This invention relates to underground thermal energy storage systems using water as the storage medium for power production and heat supply in conjunction with thermal power plants. Tire invention also relates to the construction of storage systems and hot-water power and heat internet to efficiently manage the demands and supplies of power, heat, and water.

Background of the Invention

Energy storage for power and heat is an essential part of renewable energy strategies. Although the development of renewable energies has been underway for decades, they are still playing a supplementary role in total energy supplies, and more than 80% of global energy consumption is still coming from fossil fuels. It was predicted that to successfully fight against the global warming trend, major fossil fuels, such as coal, oils, and natural gases, must be displaced by renewable energies. Solar and wind power are the primary renewable energies being developed because of their abundance and lower costs with limited environmental consequences. However, their inherently intermittent nature and seasonality are hindering their potential to displace or phase out fossil fuels, and the related energy storage is becoming a bottleneck for the future development of renewable energies.

The cunent energy storage systems for power include but are not limited to, pumped-storage hydropower (PSH), compressed-air energy storage (CAES), thermal-energy storage materials such as molten salts, batteries for renewable power storage, hydrogen technology, and flywheels (EESI, 2019). However, not all storage systems above are feasible for utility-scale storage from the viewpoint of long-term storage capability, large scale, resource availability, efficiency, costs, and environmental impacts.

Pumped-storage hydropower (PSH) uses the electricity generated by power plants to pump water to an upper reservoir from a lower reservoir to convert the electrical energy to the potential energy of water at a higher elevation. When renewable energy sources such as solar or wind power arc not available, the stored water is used to generate power through a hydroelectric power plant to convert the potential energy to electricity. The PSH uses water as the storage medium that is natural and abundant as well as clean and renewable. Additional advantages of the PSH include the ability to store a large amount of energy with a relatively high cycle efficiency of about 80%. Major disadvantages include low power storage density, safety considerations, land-use conflict with residents, and high initial investment as well as difficulties in securing permits and construction, which may lead to decade-long development timelines and billion- dollar price tags. Hydraulic dams must hold back large volumes of water at higher elevations and are consequently subject to the risks of construction failure, natural disasters such as flooding and earthquakes, or sabotage. If the water surface is not covered, the depletion of water in the reservoir under drought conditions could occur.

Compressed-air energy storage (CAES) has the advantages of providing significant energy storage at relatively low costs and having great flexibility to secure significant load management at the utility or regional level. The disadvantages include low storage efficiency of about 40-70%, meaning a substantial amount of electricity input is lost and unable to be converted back into electric power. Another disadvantage is the low air density for storage. To overcome this problem, the air may need to be compressed to high pressure and elevated temperature. The elevated temperature could cause significant heat loss during the storage time, and heating through the burning of fossil fuels such as natural gas may be required in the expansion process to avoid freezing conditions at the exit of the turbine.

Molten salts, such as the solar fluid of the eutectic mixture of 60% sodium nitrate and 40% potassium nitrate, have the advantages of stable operation and relatively lower costs and have found a niche application in high temperature concentrating solar power (CSP) and thermal energy storage. One of the disadvantages of molten salts is their low energy storage density compared to fossil fuels. Therefore, for molten salts to play a significant role in replacing fossil fuels, a large amount is needed. If they are clean and renewable as well as abundant, their use may not be a problem. However, molten salts are hazardous materials (harmfid and oxidant) that are mainly used in fertilizers and explosives, and it is unclear if their health and environmental impacts could permit their large-scale uses. Also, they are largely mined in certain countries, and it is unclear if large mining industries need to be established worldwide for large- scale uses.

Batteries as utility renewable -energy storage have the advantages of high energy density, convenience for installation, and great performance in terms of fast energy storage from and release to the power grids. However, their uses as utility energy storage have to compete for resources with the uses for electric vehicles (EVs). Although the uses of batteries for utilities have seen fast growth for the stability of power grids in recent years, the predominant uses or primary markets are in the EV industry, which is considered to be the major approach to electrifying automotive vehicles through the power supplies from renewable energy sources to displace the consumption of oils. Even at the early stage of the EV industry, serious batten or battery-material shortage has emerged. To maintain the high growth rates of EVs in the coming years, mining capacity for critical minerals, such as cobalt, nickel, and lithium, must be increased significantly, and many more batteries manufacturing facilities must be built, which could cause greenhouse gas emissions and environmental impacts as well as the availability issues of mining resources. Additionally, the long-term environmental impacts of battery disposal and recycling are uncertain. If batteries are to take a primary role in utility renewable power storage, they could face much more serious challenges; hundreds of times the batten capacity being used for EVs may be needed to store power for utility renewable power if fossil fuels are to be displaced. Furthermore, the costs and lifetime of the batteries could be serious issues. The overall costs of utility renewable power storage based on batteries could be much more expensive than other utility-scale power storage systems.

Based on the above brief discussions, current energy-storage technologies may have their unique places in renewable energy storage and have found their niche or special applications. However, it is believed that they might not be able to serve as the backbone or base load of renewable energy storage to displace the uses of fossil fuels.

Summary of the Invention

To serve as the storage medium of a backbone renewable energy storage system to displace or phase out fossil-fuel-based power and heat, the medium must be natural substances and abundant as well as renewable and clean. The medium should also be readily available without involving mining operations and have a relatively high density in a liquid form for transportation. Even if it could not match the energy density of fossil fuels, the characteristics of abundance and renewability could well compensate for the energy density mismatch. It is also understandable that once the fossil fuels are displaced, most of the thermal energy-related power (such as combustion-related power), as well as industrial heat used at high temperatures (such as processing heat and heat for mineral transformation) must be electrified. As a result, the global electricity demand could well be more than doubled, and a lot of new renewable energy power plants may be built. It is hypothesized that storage systems using hot water as a thermal energy storage medium in conjunction with thermal power plants (Cao, 2022a) that use hot water as an energy source could be the only solution to satisfy all the requirements above and be able to function as a backbone of renewable energy storage to provide both reliable energy storage and power production. The hot water herein is preferably in a liquid form. However, the hot water could also be a two-phase liquid-vapor mixture or superheated vapor although the energy storage density may be lower. The hot water storage could be underground, in- ground, or above ground. However, the underground storage, including in-ground storage with top cover and thermal insulation, could eliminate or minimize the risks of hydraulic dam failure, natural disasters such as flooding/earthquakes/sabotage, land use conflict with residents, the potential to disrupt river ecosystems, and depletion of water in the reservoir under drought, which may be normally associated with the PSH. Also, the underground storage could use earth soils as natural and effective thermal insulation to maintain the desired temperature of the hot water for years. Finally, the temperature of the hot water should be low or moderately high to accommodate large storage volumes for utility-scale power production and avoid high structural costs related to high storage pressure.

It is, therefore, a major objective of this invention to provide utility-scale (large) underground hot water storage (USUHWS) for power production or heat supply to enable renewable energy sources to produce power or supply heat reliably and continuously with minimized interruption and impact by weather conditions. Said USUHWS could retain the thermal energy content of the stored water for a long time, and as needed, provide the hot water to thermal power plants as a heat-source fluid to generate power on utility scales over extended periods. Another major objective of this invention is to provide hot-water power and heat internets to interconnect storage systems, power plants, heat sources, and various water and heat users to efficiently manage the demands and supplies of power, heat, and water. Yet another major objective is to employ the USUHWS to collect, store and supply fresh water for communities. Yet another major objective is to provide means to construct USUHWS including the use of Tunnel Boring Machines (TBMs).

Brief Description of the Drawings

FIG. 1 is a schematic top-sectional view of a utility-scale underground hot water storage reservoir according to an embodiment of the subject invention, showing interior supporting structures (or walls) for the roof of the container;

FIG. 2 is a schematic vertical sectional view of a segment of the hot-water storage reservoir in FIG. 1 with earth-soil insulation on top of the water container;

FIG. 3 is a schematic vertical sectional view of a segment of the hot-water storage reservoir in FIG. 1 with engineered insulation on top of the water container;

FIG. 4 is a schematic vertical sectional view of multiple underground hot water storage tanks for higher- temperature applications;

FIG. 5 is a schematic vertical sectional view of a tunnel hot water storage facility in a mountain or hill;

FIG. 6 is a schematic vertical sectional view of an underground tunnel hot water storage facility constructed using Tunnel Boring Machines (TBMs);

FIG. 7 is a schematic illustration of an underground pipeline transporting hot water over a distance of L_p

FIG. 8 is an exemplary illustration of a local area of hot-water power and heat internet; FIG. 9 is a schematic diagram of a larger area of hot-water power and heat internet comprising several local areas;

FIG. 10 is a schematic vertical sectional view of a storage system involving inlets and outlets for multiple functionalities; and

Fig. 11 is a schematic perspective view of folded solar collectors for land sharing.

Detailed Description of the Invention

Utility-scale (or large) underground hot water storage (USUHWS),

FIG. 1 shows schematically a top sectional view of a utility-scale (or large) underground hot water storage (USUHWS) facility for power and heat according to an embodiment of the subject invention and FIG. 2 is a schematic vertical sectional view taken along line A-A in FIG. 1. For hot water temperatures meaningfully above 100°C with a significant thermodynamic gauge pressure, the hot water 12 may be contained in a container vessel 14, which may be, but not limited to, concrete, steel-reinforced concrete, carbon fiber reinforced concrete, or metals, with a length L and a width W, as shown in FIG. 1 and a depth of water level H as shown in FIG. 2. The external walls 14 (FIG. 1) and the bottom walls 16 (FIG. 2) of the container are held against by surrounding earth soils 18. Because of the large size of the utility-scale, when under pressure, the top walls 20 (not shown in FIG. 1, but partially shown in FIG. 2) of the container may be the most vulnerable part. For this reason, a plurality of internal supports 22 (or internal walls) may be installed inside the storage reservoir, as shown in FIG. 1, which divides the reservoir into smaller segments for pressure management and supporting the roof of the container. The water 12 in the reservoir is interlinked through openings 24, as shown in FIG. 2, in the walls, and water temperature and pressure at the same level of the reservoir can be approximately uniform even though thermal stratification would occur in the vertical direction. For safety considerations, at least a pressure relief valve 26 may be installed as seen in FIG. 1. The span of the section, as shown in FIG. 2, would depend on the pressure loading and rooftop construction of the reservoir, among others. For hot water with a temperature of 100°C or lower, the construction of the reservoir can be simplified, and the costs significantly reduced due to zero-gauge pressure. As one of the choices in this zero-gauge pressure situation, the reservoir can be constructed similarly to the construction of an in-ground swimming pool and the container can be replaced by pool walls or panels. The roof ceiling of the reservoir can be flat in conjunction with sufficient support. In either the container vessel or pool construction, the land surface 28 on top of the storage system can be used for other purposes. For example, land 28 on top of the reservoir can be used for solar collectors 30 installations, as shown schematically in FIG. 2. The solar collector system 30 converts solar energy into heat to produce hot water, which may be stored by the underground hot water storage or ducted directly to a thermal power plant to produce power. Water collection element 46 (FIG. 2) may be integrated with the solar collector 30 to collect rainwater or melting snow. Some typical solar collectors will be discussed later in this disclosure.

When the solar collector system is installed on a land surface on top of an underground hot water storage system and when earth soil 32 is used as a thermal insulation means, the water collection mechanism, as representatively shown by 46, could have a function to reduce the moisture level of the insulating soils on top of the underground hot water storage sy stem to maintain an acceptably low soil thermal conductivity for higher thermal insulation effectiveness.

For maintenance and lower costs considerations, the distance between the reservoir top 20 and the ground surface 28 may be limited. Thermal insulation would be critical in this case to minimize the heat loss from the hot water to the ambient to achieve sufficient storage duration. Because the storage operation is supposed to be long-term, the heat losses through the external walls 14 and reservoir bottom surfaces can be rather small under steady-state due to the very thick surrounding soils that serve as natural and ideal insulation to these walls or surfaces. But the insulation on top of the reservoir could be critical due to the limited distance between the reservoir top and the ground surfaces. However, since the earth soil under general conditions has a rather low thermal conductance, the storage system may be adequately insulated on the top by earth soil insulation 32 with a reasonable thickness, as shown in FIG. 2. It should be mentioned that the soil thermal conductivity is sensitive to the type of soil and water content in the soil; therefore, soil selection and soil water management for the selected soil should be considered. FIG. 2 also shows schematically the deployment of a floating cover 34 at the free surface of the hot water. The floating cover minimizes the evaporation of water at the free surface and subsequent condensation at the inner surface of the top container walls 20, which cuts off a major heat transfer path between the hot water and roofing of the storage and could further increase the storage duration.

In FIG. 2, natural insulation in terms of earth soil is employed for the thermal insulation on top of the storage reservoir. However, engineered insulation layers may be employed to further reduce the heat loss from the hot water, as schematically shown in FIG. 3. There are many engineered thermal insulation techniques available in the arts. In this case, a combination of truss structures and insulation elements is created. Referring to FIG. 3, truss structure 36 transmits the weight loads on top of the reservoir to the container vessel or reservoir roofing while high-effective insulation elements 38 are filled within the truss structures between the top cover plates 40 and the top walls 20 of the container. As an example, insulation elements 38 with loose fills such as glass fibers (poured or blown) or Vermiculite (flakes), which may have a very low thermal conductance of less than 0.07 W/(m-°C), may be filled between the cover plates and the container walls. The low density of the soft fills, as low as 80 kg/m³, could also be beneficial for the deployment.

Although the details are not included in this disclosure, sample calculations have been conducted to evaluate the performance of the USUHWS systems of this invention (Cao, 2022b). For a typical size of the storage reservoir, L = W = 500 m and H = 15 m, if the average temperature of the stored water is 120°C, the temperature drop of the hot water in a heat-to-work conversion unit before being thermally recharged by a heat source is 40°C, and the thermal efficiency of the heat to work conversion unit in a power plant is 10%, then the total potential work stored by the storage system is about 1.75x 10⁷ kWh before being recharged by the heat source. This stored work potential could power a utility power plant of 100 MW capacity continuously for 175 hours or 7.30 days and for a smaller power plant of 10 MW, the operational duration is increased to 1750 hours = 73 days =2.43 months before the storage system is recharged by a heat source.

The heat sources could be any applicable renewable heat sources or waste heat from a variety of industries. However, a preferable heat source is solar energy, which is considered to be one of the most important renewable energy sources, in conjunction with solar water heating collectors. In this regard, solar collectors are employed to collect solar energy in terms of heat and store it in hot water to generate power or provide heat for industrial and domestic consumers. Non-concentrating hot-watcr heating solar collectors that could collect both direct and diffusion components of the solar flux are the starting point for the application of this disclosure. Hot water solar collectors are well-developed non-concentrating solar collectors. With its low costs, high collector efficiency, and popularity, its worldwide installation has increased dramatically in the past 20 years.

The current trend of water-heating solar collectors has moved towards evacuated tube collectors (ETC) on the basis of the heat pipe or U-tube configuration. Because of the special applications of domestic hot water and home heating, its working temperature is often around 70°C. At a higher temperature, the conventional non-concentrating solar collector may incur significant heat loss with substantially reduced collector efficiency. However, for the power generation and industrial applications of this disclosure, the collector’s output temperature can be increased to above 120°C with still good collector efficiency by using higher performance evacuated flat plate solar collectors (EFPSCs) for hot water production. The non-concentrating EFPSC includes a flat absorber within an evacuated enclosure with a top glass cover and an array of pins to support the glass cover against atmospheric pressure loading (Moss et al., 2018), and the collectors have the potential to attain sufficiently high operating temperatures up to 250°C with still reasonable collector efficiency. A competing solar water-heating collector category for a higher operating temperature is solar collectors that combine ETC and compound parabolic trough collector (CPC) technologies, or CPC-based solar collectors, which could operate in a temperature range of 70- 130°C with reasonably high efficiency.

Another renewable energy source is geothermal energy. Although a thermal power plant may directly interact with a geothermal source without involving a hot water storage system, the hot water generated through the geothermal source may store in the storage system as needed. In the case that the temperature of the hot water generated by non-concentrating solar collectors is too low for efficient power production, while the hot water generated by the geothermal source is sufficiently high, the water from the geothermal source may be used to boost the temperature of the hot water from the solar collectors. A similar boost may also be provided by solar concentrators commonly used in CSP.

Tire storage stability of the hot-water storage is also evaluated for its potential as a backbone of renewable energy storage means in terms of the storage’s potential to retain the hot water’s energy content over an extended time duration. For this purpose, a storage period is defined as a stable storage period if, at the end of the storage period, the thermal energy content of the water is still equal to or greater than 90% of the initial energy content. For the storage system shown in FIG. 2 using soil as the natural insulation medium, if the initial temperature of the hot water is 120°C, the ambient temperature is 15°C, and the effective soil insulation thickness (32) on top of the storage is 2.5 m, the corresponding stable storage period was found to be about 1.05 year (Cao, 2022b). If the top effective soil insulation thickness is increased to 5 m, the stable storage period would be 2.1 years. For the reduced distance between the reservoir top and the ground surface with even longer stable storage duration, engineered insulation layers in FIG. 3 may be employed.

The long storage duration feature of the hot water storage herein is very significant to solar energy. Solar energy can be extracted and stored at any time and in any season, and then stored heat can be used to produce power or provide heat year-round. In the current oil age, the national oil reserve has been established to deal with unexpected oil supply snags. Similarly, with the long-term storage potential of the hot water as demonstrated above, national hot water storage reserves can be established to deal with unexpected renewable energy shortfalls.

In addition, the performance of the hot-water storage system is compared to its close competitors, the pumped-storage hydropower (PSH) and compressed air energy storage (CAES), under some given conditions in terms of energy density that is generally the most critical measure for an energy storage system. The comparison results showed that the USUHWS can outperform the PSH by more than 80 times and outperform the CAES by more than 84 times.

The costs of the present USUHWS are also compared with those of battery pack systems. The results indicated that the capital costs of the hot water storage system could be two orders of magnitude lower than the corresponding battery systems. More importantly, the lifetime of the battery system is only about 10-15 years, while the underground hot water storage system could function for hundreds of years with necessary maintenance. It is well known that some ancient underground structures may have survived for thousands of years. Also, the hot water storage is almost completely clean, while the longterm environmental impacts related to mining, recycling, and disposal of battery materials could be significant.

The USUHWS could also be an integral part of the global freshwater strategy because the USUHWS would conserve the water that would otherwise be depleted due to evaporation into the open air. Understandably, the storage capacity requirement for power and heat may vary substantially under different weather conditions or demands. At times, some of the water storage capacity may be used for other purposes. Because of the thermal stratification, hot water stays at the top of the storage while cold water stays at the bottom of the storage and the USUHWS may be used to store cold water at the bottom. In this regard, some underground water storage may be part of drinking water or other freshwater water facilities to store water for residents with sufficient redundancy of storage capacity or when the hot water storage requirements for power generation and heat are reduced. Some of the storage may be used to collect rainfall water for feed water purposes or may be used to store excessive rainfall water to prevent local flooding. Yet some of the storage may provide needed water to the surrounding communities under drought conditions. Finally, if the power plant is decommissioned, the roofs may be removed, and the water storage facility would become an open-air lake. In summary, underground water storage, one way or another, is always beneficial. It causes no significant environmental impact on or risk to the surrounding communities and could benefit their ecological systems in the long run.

The cross-section of the USUHWS in FIG. 1 has a rectangular shape; however circular shapes such as cylindrical shapes may be employed, which could tolerate a higher thermodynamic pressure. FIG. 4 shows schematically a plurality of underground circular hot water storage tanks 60 for higher temperature applications including those providing industrial process heat at relatively high temperatures.

Additionally, for mountainous or hilly regions, the hot water storage facilities may be built inside the mountains or hills as tunnel hot water storage (THWS) facilities. FIG. 5 shows a schematic illustration of a tunnel hot water storage facility 70 in a mountain or hill 72 in conjunction with tunnel linings 74. In this case, popular Tunnel Boring Machines (TBMs) that are used to construct high-way tunnels and metro subways may be used to build the hot water storage cost-competitively in conjunction with related tunnel linings. Although the diameter of the tunnel may be limited to the order of 15 to 20 m, the length of the tunnel may be on the order of kilometers because of the automation of TBMs, and multiple tunnels can be made in the same mountain or hill to achieve a utility-scale storage capacity for power and heat. An advantage of tunnel storage is its circular shape which could accommodate higher pressure under a higher temperature. To reduce the water leakage out of the tunnel in case of an accident, cavity 76 may be made at the entrance of the tunnel, in combination with entrance protection and thermal insulation 78. To accommodate leaked water in an accident, surrounding protection walls 80 may be provided so that most of the water could remain in the tunnel in such an event. Also, to prevent the cavity from being flooded, it may be covered by roofing (not shown). Additionally, relief valves may be installed, and the tunnel may be tilted so that the water near the entrance is at a higher elevation than the rest of the water in the tunnel.

Furthermore, TBMs may be used to construct tunnel hot water storage (THWS) on relatively flat lands, similar to the way to construct subways. FIG. 6 show s a schematic illustration of an underground tunnel hot-water storage facility 82 with tunnel linings 84 and entrance protection and thermal insulation 86 under the land surface 88. Similar to the case in FIG. 5, for the purposes of reducing the water leakage out of the tunnel in case of an accident as well as the operation of TBMs and tunnel maintenance, a cavity 87 may be made at the entrance of the tunnel to accommodate leaked water under accident with surrounding protection walls 90, so that most of the water could remain in the tunnel in such an event. To prevent the cavity from being flooded, the cavity may be covered by roofing 92. A plurality of underground tunnel hot-water storage units may be constructed to attain utility-scale hot-water storage capacity. For example, a plurality of the underground tunnel hot water storage units may be constructed in a direction generally perpendicular to the paper and said storage facilities may share the same cavity or roofing (not shown).

Internet of Hot-Water Power and Heat

Transportation of hot water between heat sources, such as solar farms, and storage systems, as well as between the storage systems and power plants or heat/water users is essential to the operation of the renewable energy system. However, during transportation, some thermal energy associated with the hot water may be lost. FIG. 7 shows schematically an underground pipeline transporting hot water over a distance, wherein L_p is the pipeline length, T_p is the pipeline surface temperature, which can take the inlet temperature Tf of the hot water for simple conservative calculation, T_G is the undisturbed surrounding soil temperature that can take the ambient air temperature for simple calculation, d is the distance from the ground surface to the centerline of the pipe, r is the radius of the pipeline, m is the water flow rate in the pipe, c_p is the specific of the water, V is the average velocity of water in the pipe, and ATy is the temperature drop between the inlet and outlet of the pipeline. If = 100 km, T_p = Tf = 120°C, T_G = 15°C, d = 3 m, r = 0.5 m, and V= 1 m/s, it was found that AT) was less than 4°C. Since the temperature drop is directly related to the energy loss of the hot water over the distance, the above result shows that only about 3.8% energy loss would occur over a long distance of 100 km, which gives a certain degree of freedom for transporting hot water between different users over distances. The above results are based on a simple underground pipe with natural insulation. With additional engineered insulation or a deeper layout, the above heat loss can be further reduced. A simple calculation of the pumping power requirement had also been undertaken with acceptable results. In general, a higher water transport rate favors the reduction of heat loss, but a slower transport rate favors a reduction in pumping power.

So far in the disclosure, hot water storage for power generation is emphasized. However, it is also essential to store hot water to provide heat for other uses. Therefore, the hot water storage facilities should be multifunctional. In addition to the purpose of displacing fossil fuel uses for power production, multifunctionalities could make hot water storage more attractive as the costs of the storage system can be shared by several important uses. Since the energy storage density of hot water is much lower than that of fossil fuels, a large number of utility-scale storage facilities must be built but the multi-functionalities could provide additional incentives for the storage constructions with reduced unit costs. In general, hot water storage could serve many purposes including, but not limited to, the following:

1. Replace fossil fuel as a backbone of energy storage for the power grid to generate power stably and reliably whenever needed.

2. Provide process heat for various industries with reduced burning of fossil fuels.

3. Deliver hot water to metropolitans or cities for home heating that may require hot water at a relatively low temperature.

4. Provide hot water for domestic uses such as dishwashing, showering, and laundry.

5. Supply hot water for agricultural communities including food processing.

6. Store water for water treatment facilities of metropolitans or cities when surface water is not available or in short supply to produce drinking water or for other freshwater uses of the communities.

The multi-functionalities of the storage systems would require the interaction and interconnection of various sources and users through fluid-transportation pipelines. Therefore, the internet of hot-water power and heat is disclosed according to the present invention. Referring to Fig. 8, a hot-water storage facility 100 is interconnected with various users and suppliers through one-way or two-way fluid transportation lines (the lines with at least an arrow, such as 122) including, but not limited to, power plants 104, solar collector farms 106, geothermal heat sources 120, industries 108, metropolitan/cities 110, water treatment plants 114, agricultural communities 116, and water resources 118. The interconnected systems are centered around the storage facilities and may serve a given area for power production, heat supplies, and water uses, which may be referred to as a local area of hot-water power and heat internet. The interconnection could facilitate the sharing of hot water storage systems and power plants as well as the heat and water resources and make the operation of a given area more efficient and more reliable. Interconnection with industries has a second importance. In addition to providing process heat to some industries, some other industries could use water to recover some thermal energy from high-temperature waste-heat sources generated by those industries, which would otherwise be discharged into the ambient without recovery. The hot water generated through the heat recovery would be delivered to the storage system and be used to produce power or for other heat uses. It should be pointed out that additional systems may be added to the internet. Interconnections that bypass the storage facility, such as the direct connection between the solar farm and industries and the direct connection between the power plant and geothermal source in FIG. 8, can also be included. Also, the internet could be centered around a different system other than the storage system. Furthermore, several local areas may be interconnected through pipelines to form a large area, as schematically shown in Fig. 9. The interconnection could facilitate the sharing of hot water storage systems and power plants as well as the heat and water resources and make the operation of a large area more efficient and more reliable.

Although at the higher end of temperatures, the process heat for industrial uses could be largely electrified, oil, molten salts, and other thermal energy storage (TES) materials or phase-change materials (PCMs) may be used as a storage medium as they may not require high storage pressure to reduce the costs. The use of heat at elevated temperatures for industries should be more economical than the use of electricity. As a result, the hot water storage system may include some oils, molten salts, or other TES materials and PCMs as high-temperature storage media to address the demands more fully for industrial heat. Also, concentrating solar power (CSP) may be an important component of overall renewable power generation. Therefore, TES, PCMs, and CSP may also be part of the hot-water power and heat internet although they are not explicitly shown in Fig. 8.

Storage Operation

To demonstrate the potential of the storage system, the system was treated as a uniform system under steady operation in the prior discussions of this disclosure and the inlet and outlet pipelines were not illustrated. However, the operation of the storage system is essentially a dynamic process and would involve charging and discharging for multiple functionalities. FIG. 10 shows schematically an embodiment of the hot water storage facility that could enable multiple functionalities under thermal stratification with the water supply and return for some of the interactions indicated in FIG. 8. Referring to FIG. 10, during the operation of power generation, hot water 150 from a top level of the storage system is delivered to a power plant as heat-supply fluid. In the power plant, a portion of the energy content of the hot water is converted into power, and water 152 with a lower temperature exits the power plant and returns to the storage system at a lower level. Because some heat or water users such as industries require a higher temperature, water 154 at a higher level with a higher temperature may also be extracted for these users, as shown near the top of the figure.

Under the water’s thermal stratification, a temperature gradient may be maintained along with the height of the storage system, with the hotter water staying at a top level while colder water at a bottom level. The storage may be charged by a heat source such as solar energy from at least a solar farm. Because of the thermal stratification, the hot water 156 thermally charged through the solar farm at a higher temperature is admitted at a top level of the storage system. One thing noticeable is the hot water admission 158 from the waste heat recovery of some industries, which would help to thermally charge the storage system in addition to the solar charge. The colder water 160 at a lower level of the storage is pumped from the storage to the solar farm to be thermally recharged to sustain the admission of hot water from the solar farm into the storage. Even if the power generation may be continuous, the solar energy charges may be intermittent. So long as the charged amount would balance or exceed the discharge for power generation, the higher temperature of the reservoir may be maintained at top levels. When energy input from the solar farm exceeds the extraction from the storage, the thickness of the top water layer with a higher temperature would grow toward the bottom of the storage reservoir. If this process would continue over an extended period, the entire reservoir could approach a relatively uniform temperature of the charge temperature. During the times when solar energy is not available, the power plant could continue to generate power at a relatively constant input temperature to the power plant from the storage system, although the thickness of the top higher temperatures layer in the storage would continue to decrease while the thickness of the colder water layer would grow near the bottom of the reservoir. Some of the hot water extracted from the storage may be consumed and will not return to storage. For example, some of the water vapor may be released into the atmosphere with the exhaust stream out of the power plant, and in addition to producing power, a power plant may supply heat to various users through hot water 162, as shown on the left of the storage system. For these and other water supply needs, water 164 may be transported from water sources to the storage system near the bottom of the storage as shown in FIG. 10. Since the water temperature near the bottom of the storage may be near the ambient temperature, water 166 may be extracted from the storage, as shown near the bottom of the storage, and be used as the cooling water of the recovery unit in a thermal power plant to recover both water and heat from the exhaust stream before it is released into the ambient. After the water and heat recovery, the water exiting the recovery unit may be directly used for heat-related needs, such as those for domestic or agricultural community hot water, home heating, or industrial processes at a lowcr-cnd temperature. Alternatively, the water 168 from the recovery unit may be ducted into the storage system, as shown on the right side of the reservoir, for future uses. When needed, the returned water could be extracted for different uses from the storage as shown on the left side of the reservoir (162). If those uses are not needed over an extended period, water 166 may be used for other purposes such as supplying water to a drinking water treatment plant, as shown near the bottom of the storage. It should be pointed out that not all water transportation pipelines should be thermally insulated as shown in FIG. 7. For example, if the water being delivered from the storage has a temperature higher than desired, under the conditions of no thermal insulation, slower flow speed, and smaller pipe diameter, the water may be cooled without involving water-loss evaporation on the way to a water treatment facility or distribution center.

As discussed earlier in this disclosure, underground water storage may be part of a freshwater strategy. Additional storage capacity near the bottom of the storage reservoir may be provided for this purpose to store the collected freshwater and deliver it to the users when needed. In this case, the storage capacity for freshwater is an added-on capacity that could share the costs with the need for power and heat. Compared to the stand-alone underground storage for freshwater, the unit storage costs of the present method would be drastically lower.

In FIG. 10, some water supplied to and extracted from the storage reservoir is from the sides or bottom. How ever, this is for the convenience of demonstration; all water supply or extraction could be undertaken from the top of the reservoir. For simplicity, not all the storage functionalities are demonstrated in FIG. 10, but additional functionalities can be similarly added. During the day when solar energy is available, some hot water from the solar farm could bypass the storage and be directly supplied to the power plant or industries to enable higher temperature uses without concerning the pressure limitation of the storage system. However, this operation mode may miss the stability advantage enabled by the storage system.

During the initial hot-water filling into the storage or over an extended time period of operation, gases may accumulate near the top of the storage system. To avoid increased pressure due to the accumulation of gases, a degassing valve 170 may be installed near the top of the storage system, as shown in FIG. 10, which may be periodically opened to release gases into the ambient. The degassing valve may also be used as a safety valve to regulate the pressure inside the storage system.

It should be mentioned that thermal stratification and related temperature distribution in the storage system may be complicated and vary with time. The above discussion on the location of the inlet or outlet pipes with intended specific applications may not be accurate, and the locations of water admission into or discharge out of the storage system may change based on the operational conditions or needs. An inlet or outlet port of the storage system may be shared by different applications through bypassing valves outside of the storage vessels or a single port may be shared for both inlet and outlet purposes (not shown). The general strategy for storage management is to use the thermal energy of the hot water at a higher range of temperature to generate power or provide process heat for industries that require a higher temperature and use thermal energy at a lower temperature for lower temperature applications. Under emergencies, such as extreme weather conditions that render renewable energy sources mostly unavailable for an extended period, all other operations at a higher temperature other than power production may be stopped, and the storage system would mobilize its energy content to generate more electricity.

It should be emphasized that the demonstration related to FIG. 10 is to illustrate multiple functionalities of a storage facility with a single reservoir under thermal stratification. However, a storage facility may be divided into several subsystems and each of which is dedicated to storing hot water at a given temperature range. A higher temperature would incur higher costs because of a higher storage pressure for structural consideration, but a lower temperature would incur lower costs, and the total costs of the entire storage facility could be reduced if the higher-temperature water could be confined to a smaller storage volume. Also, one of the subsystems may be used to store non-water storage media such as oils or molten salts, which could enable storage at much higher temperatures with a reduced structural penalty.

It should be noted that although this disclosure emphasizes utility-scale power production and heat supply, the storage system of this invention can be used for distributed power generation or heat supply systems.

To increase the thermal storage capacity of the storage system, some pebbles, such as riverbed granite pebbles, may be added to the storage reservoir, although not shown herein.

Land Sharing of Solar Thermal Collector Systems

According to Cao (2022a), an enormous amount of hot-w ater storage may be needed after the fossil fuels are displaced. If the energy needed for hot-water production is from solar energy, the installation of solar thermal collectors for water heating may cause land use conflict with other purposes. As discussed earlier in this disclosure, the commonly used solar thermal collectors are non-concentrating collectors, including evacuated tube collectors (ETCs), such as those with heat pipe or U-tube configurations, evacuated flat plate solar collectors (EFPSCs), and combined ETC and compound parabolic collector (CPC) systems. These collector systems may have a simple structure of generally flat geometries with deployment flexibility and quicker startup because they deal with water at low temperatures. To overcome potential challenges of land uses, land sharing may be considered as outlined below:

1. Solar thermal collector panels may serve as the roof of a greenhouse wherein some of the space between individual evacuated tubes may be open to the greenhouse, so that solar energy may be supplied to the greenhouse through the openings, while tire solar collector continues to generate hot water. Also, the backside surface of the collector panels may dissipate heat into the air inside the greenhouse for winter warming. When needed, the openings may be covered with glass or plastic film to further increase the temperature of the greenhouse in the winter. Additionally, the above-mentioned solar panels may be the roof of a house or a solar house wherein the backing of the evacuated tubes may be a roof material or glass.

2. Because the USUHWS can retain the thermal energy of hot water for years with small degradation, solar energy can be extracted and stored at any time in a year, which paves the way for land sharing or dual use. Land sharing means the use of the land for power generation/heat supply as well as for agriculture, grass, or other non-hot water-related purposes. When the availability of the solar energy source is low, such as under cloudy, rainy, or snowy conditions, in the evening or early morning, or when hot water is not needed, the solar collectors can be folded so that the solar beams may be used for purposes other than hot water generation, as shown schematically in FIG. 11. The folding position as shown in the figure could also have the benefit of thermally insulating the collector panels; therefore, the collectors could ramp up work relatively quickly when they are redeployed.

3. If over an extended time period, land use has a higher priority than hot water production, the solar collectors may be pulled to a parking place, lowered to an underground position, or remain in the same place but in a folded, vertical position. Also, underground pipelines may be installed to accommodate the water supply into the solar collector and the hot water output of the solar collector.

The examples and embodiments described herein are for illustrative purposes only and various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and pundew of this application. All patents and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.

References Cited

Cao, Yiding (2022a). “An Ultimate Solution to Phasing out Fossil Fuels - Part II: Air-Water Thermal Power Plants,” Frontiers in Heat and Mass Transfer (FHMT), 19 - 2. https://www.researchgate.net/publication/363281042

Cao, Yiding (2022b). “An Ultimate Solution to Phasing out Fossil Fuels - Part I: Utility-Scale Hot Water Storage (USHWS) for Power Production and Heat Supply,” Frontiers in Heat and Mass Transfer (FHMT), 19 - 1. https://www.researchgate.net/publication/363147927

EESI - Environmental and Energy Study Institute (2019). Fact Sheet - Energy Storage. https://www.eesi.org/papers/view/energy-storage-2019

Moss, R.W., Henshallb, P., Aryac, F., Shirea, G.S.F., Hydec, T., and Eames, P.C. (2018). “Performance and operational effectiveness of evacuated flat plate solar collectors compared with conventional thermal, PVT and PV panels,” Applied Energy, Vol, 216, pp. 588-601. htps://doi.Org/10.1016/j.apenergy.2018.01.001

Claims

What is claimed is:

1. A constructed utility-scale (large) underground hot-water storage system comprising: hot water in at least one of the following states: liquid, liquid-vapor two-phase mixture, and superheated vapor; at least a thermal insulation system; and at least a communication port linked to at least a thermal power plant to supply hot water to the power plant as a heat source of the power plant, said power plant converting thermal energy content of the hot water into power.

2. The utility-scale underground hot-water storage system according to claim 1, wherein said storage system further stores hot water for at least one of the following purposes: process heat for industries, home heating, and domestic uses including at least one of the following: dishwashing, showering, laundry, and agricultural communities including food processing.

3. The utility-scale underground hot-water storage system according to claim 1, wherein said storage system further stores water for at least one of the following: water treatment facilities, cooling water for heat or water recovery units of power plants, excessive rainfall water, and freshwater for the communities.

4. The utility-scale underground hot-water storage system according to claim 1, wherein said storage system is interconnected through fluid transportation lines to users and suppliers to form a hot-water power and heat internet, said users and suppliers including power plants, solar farms, and at least one of the following: geothermal heat sources, industries, metropolitans, cities, water treatment facilities, agricultural communities, and water resources.

5. The utility-scale underground hot- water storage system according to claim 1, wherein said thermal insulation system is earth soils.

6. The utility-scale underground hot-water storage system according to claim 1, wherein said thermal insulation system is a combination of truss structures and insulation elements, said truss structures supporting the weight loads on top of the storage system while insulation elements being fdled within the truss structures.

7. The utility-scale underground hot-water storage system according to claim 1 , wherein said storage system is a tunnel storage system and is constructed using Tunnel Boring Machines (TBMs) into at least one of the following: a mountain, a hill, and the underground of a land.

8. The utility-scale underground hot-water storage system according to claim 1, wherein said storage system receives hot water from at least one industry for use by power plants or other heat users, said hot water recovering waste heat generated by said industry.

9. The utility-scale underground hot-water storage system according to claim 1, wherein said water is thermally charged or recharged by solar collectors.

11. The utility-scale underground hot-water storage system according to claim 9, wherein said solar collector system is installed on a land surface on top of an underground hot water storage system and earth soil is used as a thermal insulation system, wherein a water collection mechanism is integrated with said solar collector system for at least one of the following purposes: reducing the moisture level of the insulating soil layers on top of the underground hot water storage system and accumulating water for storage.

12. The utility-scale underground hot-water storage system according to claim 9, wherein said solar collectors are folded and are in a substantially vertical position to share land for purposes other than generating hot water.