Onboard thermal-energy-storage-based power systems for green vehicles

Abstract

This invention relates to onboard hot-water-storage-based power systems for vehicles. In this system, sufficiently high-temperature water is stored onboard an automotive vehicle, and thermal energy it contains is converted into mechanical work via a heat engine to propel the vehicle without combustion. Hot-water-powered vehicles offer a truly renewable and clean transportation solution and represent a nature-based strategy (MBS) for addressing global warming. Beyond conventional road vehicles, the hot-water power system is also applicable to off-road and specialized platforms, including agricultural machinery, short-haul marine vessels, construction and mining equipment, and rail transportation. Given that the transportation sector is a major contributor to global greenhouse gas emissions, hot-water-based vehicles are envisioned as a transformative path toward long-term sustainability, with substantial positive impacts on both the environment and the broader transportation industry.

Description

[0001]Onboard Thermal-Energy-Storage-based Power Systems for Green Vehicles This application is a continuation-in-part of U.S. Provisional Patent Application No.63 / 639,987, filed on April 29, 2024. Field of the Invention This invention pertains to vehicles powered by an onboard thermal energy storage system, and to the associated systems for converting stored thermal energy into mechanical power. The thermal energy storage system may be charged using heat derived from renewable energy sources or from electricity. Background of the Introduction By any measure, new battery technologies have achieved remarkable success in transitioning passenger cars to battery electric vehicles (BEVs) over the past decade. Nevertheless, the electrification of diesel- fueled vehicles, such as heavy-duty trucks and buses, has encountered more challenges. Consider semi- trucks, for instance. Presently, only a handful of electric semi-trucks are on the market, primarily tailored for short to medium-range operations. These trucks are predominantly deployed in scenarios where they can operate for a limited duration before returning to a centralized hub for recharging, rendering them suitable for urban deliveries or regional distribution tasks. Tesla is a pioneer in developing electric semi-trucks having an extended range with its Tesla Semi, claiming two versions with estimated ranges of 483 km and 805 km on a full charge. These ranges would allow for longer operation times that could approach 8-10 hours, depending on driving conditions and loads, potentially meeting the requirements for longer hauls. Tesla first unveiled the Tesla Semi in November 2017, with initial production and delivery dates announced for 2019. However, the Semi's production had been delayed multiple times. Until 2023, Tesla had only made a few deliveries, such as those to PepsiCo for trial purposes. Complete trial information may not be disclosed or available, but some initial disclosure may have indicated that the use of EV Semis is most advantageous for short-range travel of about 100 miles (161 km) from the company's distribution warehouse in Sacramento to where it will deliver beverages. There are companies other than Tesla working on electric trucks, such as Nikola, Volvo, Daimler, and others, albeit with varying degrees of success and market readiness. The development process of the Tesla Semi may represent a more significant challenge compared to passenger EVs. One of the biggest challenges may be related to over-sized battery packs for semi-trucks. Analysts and industry experts have estimated that the Tesla Semi would require a battery capacity of somewhere between 600 kWh for 483 km to 1,000 kWh for 800 km, depending on factors such as the efficiency of the truck, the average speed, cargo load, and driving conditions. If the battery capacity for a passenger car is 80 kWh, 1,000 kWh for a semi-truck would be 12.5 times as much as that for the car. If the pack for the car would cost about $15,000 to $20,000, unless per kWh for the semi is significantly reduced, the cost for the battery alone could be as high as $250,000, which is compared to the cost of a diesel engine system of only about $20,000 to $30,000 of a semi-truck. While the cost of individual battery cells of a battery pack of a semi may decrease due to many more cells installed, the cost per kWh at the pack level may not decrease proportionally due to additional integration and system-level requirements, which may include Cell Integration Complexity, Thermal Management, Structural Considerations, and Charging Infrastructure. As EVs are gaining a stronghold in passenger car markets, the debate on their effectiveness in reducing greenhouse gas (GHG) emissions and impacts on the environment intensifies. At present, some general mainstream consensus is that EVs can reduce overall GHG emissions by 50 to 60 percent compared to corresponding internal combustion (IC) engine cars. While further improvement in the future may be possible, the 40-50 percent of the emissions of IC engine cars incurred by EVs may be still too high for fighting against global warming due to the strong base of IC engine emissions. However, the debate on the environmental impacts of EVs is most critical and negative due to grave environmental and human rights concerns. Cited negative environmental impacts of EVs include long break-even points for GHG emissions; the extremely water-intensive practice of lithium production, which causes heavy water depletion in mining areas, as well as contamination of water resources and respiration-related health problems; a devoid of life and contamination over large areas surrounding the mining sites for nickel and cobalt; and only about 5% of the world’s total batteries being currently recycled because of the cost and the rather long process, which result in batteries being disposed of in landfills. In short, the current BEV may be a greener product but may not be an environmentally friendly and sustainable product. It is widely acknowledged that the transportation and industrial sectors represent vast markets with room for numerous competitors, each offering unique performance advantages. Battery Electric Vehicles (BEVs) have demonstrated their suitability for relatively short-distance travel and light-duty operations. However, their prospects for heavy-duty and long-range travel are less certain due to concerns such as range anxiety and high costs. Recent consumer preferences seem to favor gas-battery hybrid vehicles, potentially posing a challenge to BEVs' aspiration of becoming fully eco-friendly vehicles. This raises the question: Is there an alternative that is even more environmentally friendly without entailing environmental drawbacks or high costs? Summary of the Invention A primary objective of this invention is to provide onboard hot-water storage power systems for vehicles, wherein hot water at sufficiently high temperatures is stored onboard and used to generate mechanical work via a heat engine to propel vehicles without combustion. Such hot-water-powered vehicles represent a genuinely renewable, clean, and nature-based transportation solution, effectively addressing global warming challenges. Suitable vehicles include trucks, vans, buses, cars, agricultural machinery, short-haul marine vessels, construction and mining equipment, and rail transportation. Given that transportation significantly contributes to greenhouse gas emissions, hot-water-powered vehicles can serve as a sustainable alternative, positively impacting the environment and transportation industries. Another key objective is to introduce an air-water thermal power engine comprising a direct-contact heat- mass exchanger (DCHME), a compressor with internal cooling, and an expander. The DCHME substantially boosts the intake mass flow rate and energy content for the expander, while internal cooling notably reduces the compressor's power consumption, maintaining compressed air temperatures near ambient at the DCHME inlet. Together, these innovations greatly enhance net power output and thermal efficiency of the onboard heat engine. A further objective of this invention includes providing an alternative onboard storage system utilizing non-water storage media and corresponding operational methods, serving as an option to complement or substitute the hot-water-based power system. Brief Description of the Drawings Fig. 1: Schematic representation of an onboard hot-water storage system ^ Fig. 1a: Delivery process ^ Fig. 1b: Filling process Fig. 2: Diagram of a hot-water storage system featuring an electric heater module positioned near the bottom of the storage vessel. Fig. 3: Schematic of a semi-truck equipped with a hot-water storage module ^ Fig. 3a: Side view of the truck ^ Fig. 3b: Cross-sectional view of the storage module along plane A–A, showing insulation ^ Fig. 3c: View of a pressure cylinder within the storage module Fig. 4: Illustration of a large hot-water storage tank at a filling station, where electricity is used to heat water for vehicle refueling. Fig. 5: Schematic of a bus integrated with a hot-water storage module. Fig. 6: Schematic of a van incorporating a hot-water storage module. Fig. 7: Top view of an SUV with an integrated hot-water storage module. Fig. 8: Diagram of a sedan with a rear-mounted hot-water storage system and heat engine. Fig. 9: Schematic of a Rankine cycle-based power system utilizing onboard hot-water storage to generate steam for power production. Fig. 10: Schematic of an onboard hot-water storage power system based on the enhanced Brayton cycle (air-water thermal power). Fig. 11: Diagram of a compressor–expander system incorporating mist injection and recovery for internal compressor cooling. Fig. 12: Schematic of an onboard thermal energy storage system using a heat exchanger to transfer heat from internal storage media to the heat engine’s working fluid. Detailed Description of the Invention Water is the best natural thermal energy storage medium that can store a large amount of thermal energy at relatively high temperatures in an onboard storage system of a vehicle. The thermal energy can then be extracted from the onboard storage and used to produce power through an onboard heat engine or power plant to drive the vehicle without involving any combustion. While conventionally modeled as heat engines, internal combustion (IC) engines in automobiles are more accurately described as chemical engines due to combustion. Similarly, the battery pack in a battery electric vehicle (BEV) operates as a chemical engine, converting stored chemical energy into electricity through battery chemistry to propel the vehicle. The hot water can be produced by a renewable energy source such as solar energy, thus, a vehicle driven by the thermal energy from an onboard hot water storage system will be a true clean and renewable vehicle. Figure 1a shows schematically an onboard hot-water storage vessel with air being substantially evacuated (although perfect evacuation is not necessary) so that water in the vessel is substantially under a saturated condition with vapor staying at the top of the vessel. Although the vapor space may vary, its mass may be negligible compared to liquid water because the vapor density is normally three orders of magnitudes lower than that of liquid water. Hot water with a high content of thermal energy in terms of enthalpy ℎ^is extracted from the top of the vessel to produce power through a heat engine which will be described later in this disclosure. In the heat engine, a significant portion of the thermal energy of the hot water is converted into mechanical energy, and water temperature is substantially reduced. The water with reduced temperature, referred to as cold water, may return from the engine in terms of enthalpy ℎ^and enter the vessel from the bottom of the vessel. Due to the natural phenomenon of thermal stratification, the hot water always stays at the top while the cold water is at the bottom. Because the thermocline is relatively thin, it is not shown in the figure. As the operation continues, more hot water is extracted while more cold water returns, which is accompanied by the shrinking of the hot water layer at the top and the thickening of the cold water layer at the bottom. When the extraction of hot water is near its completion, most of the space in the vessel may be occupied by cold water, and a refilling process through a hot water station (or other means) may be needed. Figure 1b shows schematically a hot water refilling process, where hot water at ℎ^is filled into the vessel while the cold water at ℎ^is withdrawn from the vessel. The hot water may come from the top of a storage station, while the cold water is pumped to the bottom of the storage station to be thermally recharged. In some particular situations, the hot water may directly come from a renewable energy source such as a solar thermal collector system, while the cold water out of the vessel may be directly sent to the thermal solar thermal collector system to be thermally recharged. Under normal conditions, the hot-water vessel is refilled using a hot-water source. However, if the hot water is unavailable or a vehicle is left at home for a long time and the thermal energy level in the vessel is too low to go to a refill station, the vessel may be thermally recharged using electricity. If the hot water temperature is not sufficiently high, electric heaters can also be used to boost the energy content of the onboard storage system. Figure 2 shows schematically a storage vessel with the installation of an electric heater module near the bottom of the vessel. The electric heater module can resemble domestic water heating systems with straightforward and rapid operation. To evaluate the feasibility of onboard hot-water storage for vehicles, its potential for thermal and work storage density per unit mass is calculated, which is one of the most important gauges for energy storage systems. Referring to Fig. 1b, for the hot-water filling process, the commonly known uniform-state, uniform-flow process model is employed for the control volume (CV) enclosing the storage vessel:∆^^^ = ^^^^^^ − ^^^^0^ = ^^^ + ^^ℎ^ − ^^ℎ^ (1a) process, ^^^^0^ is the thermal energy of the cold water stored at the beginning of the charge process, ∆^^^is net thermal energy charged into the storage system during the process, ℎ^is the enthalpy of the hot water being charged into the vessel, ^^is the mass of the hot water charged into the vessel during the process, ℎ^is the enthalpy of the cold water discharged out of the vessel, ^^is the mass of the cold water discharged out of the vessel, and ^^^is the heat loss from the vessel to the ambient. Since the filling process is short, the heat loss is negligibly small with a moderate insulation layer thickness. If the mass of the hot water entering the vessel is the same as the cold water leaving the vessel in the process, ^^=^^ = ^, then:∆^^^ = ^^^^^^ − ^^^^0^ = ^^ℎ^ − ℎ^^ (1b) ∆^ Thermal energy storage density (kJ / kg) =^^^= ℎ^ − ℎ^ = ∆ℎ (1c)The work storage density is then in terms of the thermal-to-mechanical conversion efficiency of the heat engine, ^^^: Mechanical work storage (kJ / kg) = ∆^ = ^^^ × ^ℎ^ − ℎ^^ (2)As a calculation sample, the hot water being charged is assumed to be at 300oC with an enthalpy of 1344.0 kJ / kg, while the annual average temperature of returning cold water is at 30oC with an enthalpy of 125.8 kJ / kg, with both of the enthalpies being taken directly from the saturated water tables. The thermal energy storage density is then calculated as follows:∆ℎ = ℎ − ℎ = 1344.0 - 125^ ^ ^" ^'^ ^^ .8 =1218.2^! = 1218.2 ^! ( #$%%&^ = 0.338 ^! (3) Although the energy storage density of battery pack of an EV may vary, the storage density of a mainstream product is about 0.212 kWh / kg for a battery module, which includes more than 18,000 cells. A battery pack may comprise more than a dozen modules, and its energy density may be close to or slightly lower than that of the module. Compared to the data in Eq. (3), the thermal energy storage density of hot water at about 300oC is much higher than the energy density of the battery module. It is well documented that for vapor power systems, a thermal-to-work efficiency of ^^^= 40% at about 300oC may be achievable, and the related work storage density is:∆^ = ^^^ × ^ℎ^ − ℎ^^ = 0.4×1218.2^ ^!= 487.3 ^ ^!^ ^" #$%%&^ = 0.135^'^ ^! (4) It should be noted that the working storage is much more valuable than the energy stored by the battery, as conversion from the chemical internal energy of the battery to electricity as well as the energy conversion from the motor to vehicle wheels will incur a large amount of loss. Table 1 shows water thermal energy storage density and work-storage density according to Eqs. (1c) and (2) at different temperatures and thermal-to-work conversion efficiencies over a hot-water temperature range of 200oC to 360oC, including the results from Eqs. (3-4). Since the water is saturated, the corresponding saturated pressure and density are also available and listed in the table. It can be seen from the table that at a higher water temperature, the work storage increases significantly. However, this increase is accompanied by a substantial increase in system pressure as well as a reduction in density. Higher temperatures would also increase the weight and cost of the vessel container. For this reason, a moderately high temperature of 300oC is used as the design point for comparison. Also, it has been shown that a thermal efficiency of about ^^^= 40% is feasible, so this value of the thermal efficiency will be used for comparison. Table 1: Water thermal / work-storage densities at different temperatures and thermal-to-work conversion efficiencies based on saturated-water property data. T (oC) Ps (bar) Thermal energy Work storage Work storage Work storage Water storage density density (kJ / kg) density (kJ / kg) density (kJ / kg) density k k ^ ^ 4 ^ k3 Once the hot-water work-storage density of onboard hot water storage is determined, its wheel-work storage density will be calculated and compared to that of the EV battery pack. For vehicle applications, wheelwork storage density, which is directly used to drive the vehicle, is the ultimate gauge for the performance of any onboard storage system. It is also well known that one of the fairest comparisons is based on the life cycle of a vehicle or under the condition of longevity. It is widely recognized that an internal combustion (IC) engine can often endure beyond 20 years and exceed 322,000 kilometers of usage. For the current thermal storage-based vehicle, the engine structure may be similar to an IC engine, but without involving any combustion and fuel-related corrosion. The maximum operating temperature of around 300oC vs. a peak operating temperature of about 2000oC of a combustion engine means the present heat engine could last much longer than the combustion engine without significant performance degradation. On the other hand, one of the serious problems for a BEV is the relatively fast degradation of battery packs, which requires a warranty of 161,000 km or eight years for the battery pack. A battery pack could last much longer under certain operational conditions. According to the American Automobile Association (AAA), for longevity, it is recommended to charge an EV battery only up to 80%, and some public stations for fast charging may cut off charging once a battery reaches that level. Also, to avoid low charge levels, the battery charge level should be kept above 20%. Therefore, the buffer factor -&for longevity of battery packs should be about 60%. The wheelwork storage density of the onboard hot-water storage can be calculated by the following relation: ^.^^^ / = ^^^ × ^0 × ∆ℎ (5)where ^0is the drivetrain efficiency which may take a value of 0.92. Use 300oC as the design point, according to Eqs. (3) and (4), ∆ℎ = 0.356 kWh / kg and ^^^ = 0.40. Therefore, the hot water (hw)wheelwork storage density would be: ^.^^^ / ,^. = ^^^ × ^0 × ∆ℎ = 0.4 × 0.92 × 0.338 = 0.124 kWh / kg (6)The wheelwork storage density of the battery pack (bp) can be calculated by the following relation: ^.^^^ / ,67 = 8 × ^7. × -& (7) where e is the battery energy density, which is normally e = 212 Wh / kg = 0.212 kWh / kg, ^7.is the conversion efficiency from the battery's chemical internal energy to the wheelwork, which according to DOE is about 0.75 without considering the regenerative braking effect for comparison. This is because a hybrid vehicle option of the present onboard hot-water storage and a battery unit may be employed to enjoy the benefit of regenerative braking. Also, for highway driving of semi-trucks, regenerative braking benefits may be rather limited. The 10% loss associated with charging the battery through grid power is not included in the present calculation. -&is the buffer factor, which according to the discussion earlier in this invention is taken to be 0.6 for longevity. Therefore, the wheelwork storage density for the battery pack (bp) would be:^.^^^ / ,67 = 0.212 × 0.75 × 0.6 = 0.0954 kWh / kg (8)Comparing Eq. (6) with Eq. (8), the hot-water (hw) wheelwork storage density may be about 30% higher than the wheelwork storage density of the battery pack (bp): .;<==>,<;?.;<==>,@A%.BCD?%.%EFD .;<==>,@A= %.%EFD = 30% (9) of has not included the weight of the container walls of the onboard hot-water storage system. According to the sizing of an onboard storage module, the weight of the container walls is only about 5.6% of the weight of the water stored using carbon fiber composites for the container walls. Therefore, for an order of magnitude comparison, the result of Eq. (9) is valid without including the weight of container walls. The above comparison is based on an ideal ambient temperature of around 20oC. It was reported that in the deep winter, the wheelwork available from the battery pack storage may be up to more than 40-50% lower than that under ideal ambient temperature. On the other hand, for the onboard hot water storage system, only about 40% of the thermal energy stored has been used to produce work in the heat engine system, and the remaining thermal energy can be used to sufficiently warm up the vehicle before it is dissipated into the ambient. Additionally, it has been known that when the operating temperature of the battery pack is above 40oC, its storage capacity could fall sharply. As a result, if the ambient temperature is above 40oC, a large amount of cooling air from the onboard air-conditioning system of an EV must be generated to cool the battery pack, which could consume an unacceptably high level of power from the battery. In summary, when the ambient temperature is around 20oC, the storage density of the onboard hot-water system could match or exceed that of the battery pack. However, in the cold winter or hot summer, the onboard hot-water system may significantly outperform the battery pack. In the above comparison, the hot water can be produced from renewable energy sources such as solar energy. However, it can also be produced through electric heaters using the electricity from power grids (Fig. 2). Since electricity is an energy of higher quality compared to heat, the conversion from electric energy to thermal energy is 100%, meaning 1 kW of electric energy will be converted into 1 kW of heat. Therefore, the wheelwork storage density of the hot-water storage as well as its comparison with the EV battery pack will be the same as the case when the hot-water is produced by a renewable energy source. However, the use of solar energy or other non-electricity heat sources will have a significant advantage in energy utilization efficiency. Because the onboard hot water storage of this invention is well suited for renewable energy applications, its solar energy utilization efficiency is evaluated and compared to that of an EV battery pack. EVs using fossil-fuel-produced electricity have limited advantages in terms of reduction in greenhouse gas emissions over IC engine-based vehicles. However, it is expected that in the future, EVs could use the power-grid electricity that is largely produced from renewable energy such as solar energy in conjunction with PV panels. It is for this reason that EVs are being promoted and subsidized by governments worldwide. The solar energy utilization pathway for EVs in the future is shown below: Solar energy PV panel Power grid Battery charge Battery to wheels ^G^= 17% ^GH= 90% ^I^= 90% ^I'= 75% Currently, the ef nt the inverter efficiency of about 95%, the average efficiency is about 17%. The power grids (PG) typically have a transmission efficiency of about 90%. The efficiency of battery charge (BC) from the power grids to the battery pack is about 90%, and the conversion efficiency from battery to wheelwork (BW) is about 75%. Again, the regenerative braking effect is not included in the comparison as a hybrid vehicle option of onboard hot-water storage and a battery unit may be employed. Therefore, the solar energy utilization efficiency of EVs, ^^^, may be calculated by the following relation:^^^ = ^G^ × ^GH × ^I^ × ^I' = 0.17 × 0.9 × 0.9 × 0.75 = 0.103 = 10.3% (10)The solar energy utilization pathway of onboard hot water vehicles is shown below: Solar energy Solar collector UG pipeline Heat engine Heat engine to wheels ^^= 75% ^ = 95% ^^^= 40% ^ = 90% The hot-water ve gy gy g lar thermal collectors (SC), which typically have a collector efficiency of about 75% using concentrating solar collectors at an operational temperature of about 300oC. In addition to concentrating solar thermal collectors, non-concentrating solar thermal collectors, such as evacuated flat plate solar collectors (EFPSCs), can be employed to produce hot water to reduce initial investments and maintenance costs. One embodiment is the combination of evacuated tube collectors (ETC), evacuated flat plate solar collectors (EFPSCs), and concentrating solar collectors. The ETC normally works at a lower temperature but has a high collector efficiency and low cost. The concentrating solar collector has a higher cost, but its use is reduced, just for boosting the water to a sufficiently high temperature. The water flow path of the combined system is shown below: Evacuated tube Evacuated flat plate Concentrating Cold Hot water The hot farms or storage systems to hot-water filling stations through underground pipelines (PL). Since the pipeline is buried underground and the soil surrounding it would provide ideal thermal insulation, its thermal energy loss could be below 5% of the energy content before reaching a filling station. So, the pipeline (PL) efficiency is about 95%. The heat loss related to the filling of hot water to the onboard hot-water storage can be negligible because of the short distance and fast filling process. The biggest loss comes from the conversion of the thermal energy content of the hot water into work through a heat engine (HE). As discussed earlier, the thermal-to-work conversion efficiency may be 40%. Finally, the work developed by the heat engine is transmitted to the wheels through a transmission (TM) system that typically has an efficiency of about 90%. Thus, the solar energy utilization efficiency of hot-water vehicles (HWV) can be calculated as below: ^^'^ = ^K^ × ^GL × ^^^ × ^MN = 0.75 × 0.95 × 0.4 × 0.9 = 0.257 = 25.7% (11)Eq. (10), the ratio of ^^'^to ^^^is 25.7 / 10.3 = 2.50, of hot-water vehicles is about 2.5 times as high as that of EVs. Currently, one of the most serious hurdles for solar energy expansion is the land use conflicts for solar fields. Utility-scale PV solar projects require far more land than comparable fossil-fuel plants and solar farm projects are under attack by residents and farmers. A recent study indicated that for the EU, India, Japan, and South Korea, at 25–80% solar-energy penetration in the electricity mix of those regions by 2050, the resulting land cover changes for solar energy, including indirect effects, will not result in a reduction in carbon release, but likely cause a net release of carbon, and a key solution is to increase the solar utilization efficiency. Since land use is inversely proportional to solar utilization efficiency, when the efficiency is increased to 2.5 times the current efficiency, the land use will be reduced by 60%. Therefore, the adoption of hot-water-based vehicles will significantly relieve the land use problem to the benefit of the solar energy industry. The sample calculation for energy storage density and solar energy utilization was based on hot water at 300°C. However, this higher temperature corresponds to a relatively high operating pressure of approximately 85 bar. To lower this pressure, a moderate hot-water temperature of 240°C is considered, accompanied by a decreased thermal-to-mechanical efficiency of about 35%. In this scenario, the thermal energy storage density from Table 1 is approximately 911.5 kJ / kg (equivalent to 0.253 kWh / kg). Consequently, according to Eq. (5), the wheelwork storage density is calculated as follows: ^.^^^ / ,^. = ^^^ × ^0 × ∆ℎ = 0.35 × 0.92 × 0.253 = 0.0815 OPℎ / ORSubsequently, with battery pack would be: ^.^^^ / ,^. − ^.^^^ / ,67 0.0815 − 0.0954== −14.6%0.0954 The results above density of hot water is approximately 15% lower than that of a battery pack. However, under extreme seasonal conditions—such as cold winters or hot summers—the storage density of hot water may still equal or surpass that of batteries. Compared to operation at 300^°C, using a working temperature of 240°C significantly reduces the required pressure to about 33 bar, down from over 85 bar. This lower pressure can substantially decrease the cost of the power system, especially for storage vessels made from composite materials. Furthermore, due to the higher density of water at 240^°C, the volumetric energy storage density may improve by approximately 14% compared to operation at 300°C. Despite the fact that the wheelwork storage density per unit mass of the onboard hot-water storage may match or exceed that of the EV battery pack, the volumetric energy density or the wheelwork storage per unit volume may be significantly lower than that of EV battery packs. A quick estimation shows that the volumetric wheelwork density of hot-water storage may be less than half that of the EV battery pack, resulting in an increased size of the water storage system. This in part is due to a reduced water density at an increased temperature. For instance, from Table 1, at a temperature of 300oC, the water density is only about 712.5 kg / m3, much less than that at the ambient temperature which is about 1000 kg / m3. For this reason, in the flowing sample vehicle design and integration of onboard hot water storage systems, the negative effects of the lower volumetric energy density will be minimized. Figure 3a shows schematically a hot-water-storage-based semi-truck, wherein a hot-water storage module is installed behind the truck’s day cab or sleeper cab. Figure 3b shows a cross-sectional area of the storage module along plane A-A, which contains several storage cylinders of hot-water pressure vessels. One of the cylinders is schematically shown in Fig. 3c and is oriented vertically along with the other cylinders behind the truck cab. Although not shown, the cylinder vessel may be interconnected with other cylinders in the module so that only one fluid inlet and one fluid outlet are needed for the module. The height H of the module may be designed in such a way that the top of the module does not meaningfully exceed the corresponding top of the trailer. The width W of the storage module may also be less than or approximately the same as that of the trailer so that the frontal surface of the module does not exceed the maximum frontal area of the truck to minimize the negative effect on the performance of the truck. It is well-established in Fluid Mechanics that the total resistance against the motion of a truck is the combination of rolling resistance STbetween the tires and the road and the aerodynamic drag SU, which can be calculated by the following relation:SM = ST + SU = ^T × VR + ^B UWX"CC ,Y^"Z (12) is the truck’s maximum projected frontal area, and V is the traveling speed of the truck. The corresponding power required to drive the truck is: [ ^ B M = SM× \] = ^^T × VR + ^UWX"C ,Y^"ZC^ ×^ \](13) According to a sample calculation, the second term in Eq. (12) related to aerodynamic drag may be responsible for more than 70% of the total power required to drive the truck on the highway. Since the installation of the water storage module does not affect the frontal area which determines the aerodynamic drag in Eq. (12), the dominant aerodynamic drag would not be increased meaningfully for increased weight and size. As will be discussed, the installation of the water storage module may result in about 10% weight increase as compared to the current semi-truck based on diesel engines, resulting in a certain performance penalty through an increase in rolling resistance (First term in Eq. (12). However, because the rolling resistance represents a small fraction in the total power consumption, its contribution to the total power increase due to weight increase is almost negligible. Therefore, the storage module may have a limited effect on the performance of the truck resulting from an increase in either the weight or size of the truck. The module dimension l in the longitudinal direction of the truck (see Fig. 3a) is determined by how much water needs to be stored to satisfy the required wheelwork storage capacity, and it can somewhat increase the overall length of the truck. According to a related evaluation, the storage capacity of the module is calculated for a cylinder diameter d = 0.6 m with the dimensions of the module of W = 2.4 m, H = 3.44, and l = 1.4 m. The results show that at 300oC, an equivalent battery storage capacity of about 1201.0 kWh-battery is achieved. The calculation result herein demonstrates that the equivalent battery pack capacity of the hot-water storage exceeds 1,000 kWh-battery for long-range travel of 500 miles (805 km). Unlike the battery pack, the hot-water storage is highly scalable; a higher storage capacity can be simply realized by increasing the size of the storage module through an increase in thickness l. If the engine bay size (see Fig. 3a) is the same as that of a corresponding IC engine-based semi-truck, the increase in truck length is about l = 1.4 m. Compared to a typical semi-truck length of 22 m, the percentage increase is only about 6.4%, which may be acceptable to the truck industry. The total weight of the stored water is calculated to be about 4,289.0 kg, which is compared to a gross vehicle weight of up to 80,000 pounds or 36,320 kg in the U.S. Therefore, the weight increase is about 11.8% compared to a diesel engine-based semi-truck if the heat engine system of the water-storage-based vehicle has the same weight. A diesel engine is generally heavy up to 6,000 lbs or 2724.0 kg. However, without combustion and associated high-pressure structure, the weight of the heat engine of the present system may be somewhat lower than the diesel engine, and the overall weight penalty may be limited to less than 10%. Also, as discussed earlier in this invention, the storage density per unit mass of the water storage system may match or exceed that of the battery pack of an EV, resulting in a similar weight penalty for the EV. It should be noted that the actual performance penalty resulting from a 10% weight increase is much lower. Since the weight increase is only related to an increase in rolling resistance, which is about 30% of the total resistance for highway driving, the actual performance penalty is only about 3%. Furthermore, since the present integration of the storage module with a semi-truck, as shown in Fig.3a, does not increase the projected frontal area, which determines the aerodynamic drag, the slight increase in truck length may result in a negligible penalty on the performance. In Fig. 3a, the heat engine to be coupled with the hot- water storage module is separately located in a frontal engine bay. However, the heat engine may be installed with the storage module behind the truck’s day cab or sleeper cab, and the frontal engine bay is eliminated. For an EV without driving, a charged battery pack could retain sufficient energy content for a long time. However, a hot-water storage module would consistently lose heat even if the vehicle is not in operation. If a hot-water storage module is defined to be operational when the module still retains equal or more than 90% of the amount of the thermal energy initially fully charged, the operational critical period is thus defined as the period between full charge and the time when the module has lost 10% of the charged thermal energy. A detailed calculation shows that by using modern reflective insulation layers, such as evacuated silica power, the critical period could be as long as 41.7 days with an insulation layer thick as shown in Fig. 3b to be about 0.1 m. The density of the insulation layer is also so low that it would not increase the module weight meaningfully. Because the critical period is so long, a charge cycle could be many times shorter than the critical period, and heat loss is not a problem when the vehicle is regularly used. However, if the vehicle is parked for months without use or recharge, the water storage module may lose all the heat content. In this particular case, the module can be charged using the electric heaters shown in Fig. 2 at home or any place with available electricity before driving the vehicle to a filling station. As previously discussed, onboard water storage can be electrically charged for operation when hot water is unavailable. Although this may lower the energy utilization efficiency, it remains a viable option, converting the vehicle into a hot-water-storage-based electric vehicle, or a Hot-Water Battery (HWB) EV. If electricity from the power grid becomes sufficiently inexpensive and clean, or if hot water is produced during periods of excess grid electricity, this type of EV could become cost-effective and environmentally competitive compared to battery EVs. Figure 4 illustrates a large, underground hot-water storage system at a stationary filling station. While actively operating vehicles may not always be charged electrically at optimal times, stationary storage systems can leverage excess solar or wind-generated electricity, storing energy as hot water when intermittent renewable power would otherwise be curtailed. This approach takes advantage of low-cost grid electricity. Besides enhancing the economic feasibility of HWB-EVs, stationary hot-water storage effectively provides an innovative renewable energy storage method. During periods of oversupply from intermittent renewable sources, excess electricity is converted and stored as hot water for later vehicle use. Compared to large battery or hydrogen storage systems, stationary hot-water storage—including the process of transferring energy to vehicles—could be significantly cheaper, due to the high costs associated with battery storage and the very low energy density of hydrogen. Furthermore, compared to large-scale battery and hydrogen storage systems, underground hot-water storage is environmentally friendly, poses no fire hazards, and is well-suited for installation in densely populated areas. Figure 5 shows schematically a hot-water-storage-based bus, where a hot-water storage module is installed behind the passenger compartment of the bus. As shown in the figure, some heat engine components may be installed in the same section of the hot-water storage module. In Fig.5, an engine bay is shown in the frontal section of the bus. However, the entire heat engine system may be located in the same section of the hot-water storage module, and the frontal engine bay can be eliminated. In terms of size, the bus may be in a category similar to a semi-truck. The evaluation of the water storage module may take a similar path as did for the semi-truck and is not being demonstrated herein. Figure 6 shows schematically a hot-water storage-based cargo or delivery van, wherein a hot-water storage module is installed behind the van’s cabin. To simplify the evaluation procedure and compare the potential energy storage capacity with that of a van, the module’s equivalent battery-pack density per unit volume for the semi-truck in Fig. 3a is calculated, which is about 95.9 kWh / m3(battery pack equivalent). Consider a cargo or delivery van with a length of 6.3 m, a width of W = 2.25 m, and an external cargo space height of H = 2.4 m. The water-storage module would have the same cargo space height and width as those of the van. If the dimension of l shown in Fig.6 is 0.4 m, the equivalent storage capacity in terms of equivalent battery pack capacity would potentially be:P × ^ × ^ × 95.9 = 2.25 × 2.4 × 0.4 × 95.9 = 207.0 kWh-battery equivalent (14)The above equivalent battery capacity is compared to the gross battery capacity of 150 to 180 kWh of Rivian delivery vans. The percentage of the increase in length for the same engine bay would be 0.4 / 6.3 = 6.3%, about the same as that of the semi-truck in Fig. 3a. It should be mentioned that because the storage density per unit mass of the hot-water storage may match that of the EV battery pack, no weight penalty may incur when compared with an EV van. Again, the storage capacity of the module can be increased by simply increasing the module thickness l. Although not being elaborated, the installation of the storage modules as well as the evaluation may be undertaken similarly for pick-up trucks. Figure 7 shows schematically a top view of a hot-water storage-based sport utility vehicle (SUV), a much smaller vehicle than semi-trucks, buses, and vans discussed earlier in the invention. In this case, the hot- water storage module may be deployed between the frontal heat engine system and the cabin for the driver and passenger of the SUV. Consider a typical SUV that has a length of 5 m, a width of 1.95 m, and a height of 1.8 m. For an approximate evaluation, the thickness of the module l is assumed to be 0.5 m, the width of the module is W = 1.95 m, the same as that of the SUV, and the height of the module is H = 1.0 m. By using the same equivalent battery-storage density of the semi-truck, the equivalent storage capacity in terms of battery pack capacity would be:P × ^ × ^ × 95.9 = 1.95 × 1.0 × 0.5 × 95.9 = 93.5 kWh-battery equivalent (15)Although the above storage capacity may look reasonable because it is in the same range as some current SUV EVs. However, the penalty of length increase of the vehicle would be 0.5 / 5.0 = 10% if the engine bay length is assumed to be the same as the IC engine vehicle. This calculated percentage length increase may be tolerable, but it is much higher than that of the semi-truck or delivery van, which is around 6.0%. Since the width of a vehicle normally does not change significantly regardless of the type of vehicle, it is the height of the vehicle that may accommodate a large storage module without compromising the length of the vehicle. In the present case of an SUV when the storage module is installed in front of the driver's cabin, the top of the storage module may be limited to a position below the frontal window of the vehicle, and therefore, the height of the storage module would be much shorter than that of the semi-truck or van. Now if the size of the vehicle is further reduced to that of a sedan car with further reduced vehicle height for fuel efficiency consideration, the permissible storage module size may be further reduced, which may seriously affect its energy storage capacity and may be less competitive than a BEV. However, this problem may be significantly alleviated if the storage module and the associated heat engine system are rear-mounted. Figure 8 shows schematically a hot-water storage-based sedan car, where a hot-water storage module is installed behind the cabin. Even though the maximum height of the car is rather limited, the permissible height of the storage module may be sufficiently high to satisfy the energy storage capacity needed. Also, the heat engine system may be installed with the storage module behind the car and the trunk space is shifted to the front of the car as shown in the figure. Additionally, for the SUV shown in Fig.7, the storage module may be shifted to the back of the SUV for higher storage capacity and reduced size penalty similar to the case in Fig. 8 (not shown). In summary, because of the relatively low density of water, the onboard hot-water storage system may favor a larger-sized vehicle, but it could also work sufficiently well with a smaller vehicle. It should be noted that in addition to the road transportation vehicles described in this invention, the onboard hot-water storage system combined with heat engines can be utilized as an onboard power solution for various other applications or vehicles, including but not limited to: 1. Construction and Mining Equipment: The power systems of heavy machinery such as excavators, bulldozers, and dump trucks could be replaced by hot water-based systems. 2. Agricultural Equipment: Vehicles like tractors and combines could use hot water produced by burning agricultural waste, although various other heat sources are also possible. 3. Short-Haul Marine Vessels: Cargo ships, boats, and other marine vessels can naturally accommodate large hot-water storage tanks. Additionally, these vessels can efficiently use seawater or river water as coolant for the heat engine condenser, making them ideal candidates for adopting hot-water storage-based systems. 4. Rail Transportation: Onboard hot-water storage systems could replace diesel-electric locomotives by generating electricity to power trains. Railways have sufficient space for thermal storage, which can be recharged at stations. Compared to costly electric trains, hot-water-powered trains could offer significant cost advantages. 5. Distributed Electricity Generators: In regions experiencing frequent power outages or lacking grid infrastructure, hot-water-based power systems could serve as reliable primary electricity sources. Heat engines are essential components of onboard hot-water storage-based power systems, which can be classified into Rankine cycle systems and air-water thermal power systems (or enhanced Brayton cycle systems). Today, the Rankine cycle systems based on steam turbines are still generating most of the electricity worldwide through fossil-fuel and nuclear power plants. Major manufacturers of steam turbines include GE Vernova, Siemens Energy, and Mitsubishi Heavy Industries. One of the key features is that they can work at moderately high temperatures with high efficiency. For example, in most nuclear power plants, steam at about 300oC or below is normally produced and used to produce power. Although they are generally on a large scale on the 100 MW level and use conventional fossil fuel or nuclear heat sources to generate steam, they can be scaled down to 100 kW level and use hot water from onboard storage to generate steam and produce mechanical power to drive vehicles. In pressurized water reactors (PWRs) nuclear power plants, which are the dominant type of nuclear power plants today, hot water generated by the nuclear reactor is used to produce steam for power production. In the present application, hot water at a similar temperature level is extracted from the onboard storage to generate steam for power production. Figure 9 shows schematically a Rankine-cycle-based power system using onboard hot water storage to generate steam for power production. Since the objective herein is to demonstrate the working principle of the power system, Fig.9 does not reflect an actual arrangement of the components of the system. Referring to Fig. 9, how water (solid red line) at a temperature _^is delivered from the right side of the storage vessel into a counterflow steam generator, wherein the hot water transfers heat to the counterflowing working fluid of the power system and produces high-pressure / high-temperature vapor flow. The working fluid of the power system is also generally water although it may be separated from the water-flow loop related to the onboard storage vessel. The generated steam exits the steam generator and enters an expander such as a steam turbine. The steam expands in the expander and generates mechanical power, which is used to drive vehicle wheels (not shown) through the drive shaft. The exhaust steam or vapor-liquid mixture with low pressure and low temperature exits the expander and is ducted into a condenser, wherein the vapor is condensed back into liquid by the airflow through the condenser coils. The condensate out of the condenser is circulated back into the steam generator as the working fluid of the power system, which is vaporized into steam again to complete the cycle. After releasing its thermal energy to the power-system working fluid in the steam generator, the hot water from the hot- water storage becomes a cold-water flow stream and is pumped back into the storage vessel with a temperature of _^, which could be close to the temperature of the condensate out of the condenser. One challenge generally associated with an onboard Rankine cycle system is the condenser capacity required. However, a new system called microchannel condenser, which has been developed in recent years, could be adopted for the present systems. The microchannel condenser has the potential to improve condensation efficiency by 40% and reduce the weight by 60%, which is very beneficial to the present application. A second challenge related to the onboard power system is the potential lengthy cold startup time because the system may be filled with air before the cold start as well as the thermal inertia of system mass. For commercial transportation, this may be acceptable, but for consumer applications, the lengthy start time is not tolerable. As discussed in connection with Fig.1a, the top space is filled with high-pressure and high- temperature vapor, which can be used for cold startup. If the vapor is extracted from the top space of the storage vessel, some liquid water near the top may flash into vapor to compensate for the vapor loss in the top space to reach a new thermal balance; therefore, the vapor flow could continue. Referring to Fig. 9 again, once the control valve is open on top of the storage vessel, the vapor (red, dashed line) will be released from the storage vessel into the expander to do work without significant thermal inertia effect. At the beginning of the startup, the condenser may work at a pressure higher than the ambient pressure; therefore, the gas being pushed together with exhaust steam into the condenser out of the expander may be released by the gas release valve on the top right of the condenser. A gas discharge mechanism for the loop space inside the steam generator may also be added, but it is not shown for simplicity. Once the power system is fully started, the valve on top of the storage vessel will be closed, and the hot water from the storage vessel will be used to generate power in normal operational mode. A third issue is related to the steam turbine expander. A steam turbine expander works best in highway driving mode because it can provide a high expansion ratio for higher thermal efficiency under the condition of high-power density, resulting in a smaller size and lower weight. However, for local or city driving, steam turbines may face challenges under stop-and-go conditions because their performance depends on sufficiently high rotating speeds. Similar to the IC engine / battery hybrid vehicles, onboard hot-water power / battery hybrid vehicles may be adopted, which could overcome the stop-and-go issue while enjoying battery-related regenerative-braking benefits to significantly improve city-driving efficiency. The hybrid approach can also significantly reduce the cold startup challenge of the how water power system. After the disclosure of the heat engine based on the Rankine cycle, enhanced Brayton cycle systems based on the concept of the air-water thermal power plant are disclosed, although the air-water power in the prior arts was for utility-scale power production at low temperatures. The Brayton cycle is the foundation behind gas-turbine power plants and aircraft engines among many other applications using air as the working fluid. The basic Brayton cycle includes a compressor that admits ambient air and raises its pressure, a high-temperature heat exchanger for the air to acquire thermal energy from a heat source, an expander such as a gas turbine, wherein the air with high energy content expands in the turbine to produce power, and a low-temperature heat exchanger, wherein the exhaust air out of the turbine dissipate the waste heat to the ambient and is circulated back to the compressor to complete the cycle. Almost all gas- turbine power plants and engines practically operate on an open cycle that directly discharges the exhaust air into the ambient without involving the low-temperature heat exchanger, but the Brayton cycle is still used as a thermodynamic cycle for analyses. The biggest problem is the high-temperature heat exchanger. Unlike the steam turbine wherein water is used as the working fluid, which is an outstanding heat transfer medium, the air is a very poor heat transfer fluid, and not much thermal energy can be acquired by the air from the heat source. Practically, the high-temperature exchanger is almost always replaced by a combustion chamber wherein the fuel-burning released heat is effectively passed to the working fluid in terms of combustion gases. For the present application with no combustion, the airflow could face significant challenges without being able to effectively acquire thermal energy from the heat source under just moderately high temperatures. However, this problem can be adequately solved by replacing the high-temperature exchanger with a direct-contact heat and mass exchanger (DCHME) following the concept of the air-water thermal power plant to drastically enhance energy acquisition of airflow from the heat source. The DCHME can also increase the mass flow rate in terms of added vapor flow into the turbine to substantially increase the power output of the system under a low or moderate heat source temperature. Figure 10 shows schematically an onboard hot-water storage power system on the basis of the enhanced Brayton Cycle to demonstrate its working principle. The system includes a compressor wherein ambient air is admitted and compressed to a sufficiently high pressure. To reduce the power consumption of the compressor and maintain a low exit temperature of the compressed air, an internal cooling technique is employed (to be illustrated later). The compressed air is ducted into a direct-contact heat and mass exchanger (DCHME) or packing from the bottom (blue line). At the same time, hot water is delivered from the top of the onboard hot-water storage and enters the DCHME from the top (red line), forming two direct-contact counterflow streams along the DCHME. Within the DCHME, some of the hot water is evaporated at the interfaces between the liquid hot water and the air and the generated vapor enters the air flow stream as mass transfer along with the sensible heat transfer. Because of the vapor mass transfer from the downflowing hot-water stream, the airflow stream exits the DCHME as a mixture of air and vapor stream. The hot mixture of air and vapor then enters the expander (red line) to develop mechanical power to drive the wheels of the vehicle (not shown) as well as the compressor. The power system also includes a regenerator condenser to recover both water and heat from the exhaust air-vapor mixture out of the expander. In gas turbine analyses, the importance of the regeneration cannot be overemphasized. Numerous calculation examples have shown that for a simple Brayton cycle with an overall thermal efficiency of about 35%, the use of a regenerator could potentially increase it to over 50%. The use of the regenerator for the present application can bring similar benefits. More importantly, in gas turbine power plants, the regenerator must deal with high-temperature exhaust combustion gases which could cause a lot of material-related challenges such as corrosion, while in the present application, the regenerator could work at temperatures below 100oC. Also, in the present application, water and air are the only working fluids with no combustion gas involved, and the regenerator condenser can be a direct- contact type and has low costs and high efficiency. Referring now to Fig. 10 near the bottom, the exhaust mixture of air and vapor out of the expander enters the direct contact regenerator condenser from the bottom. At the same time, after the down-flowing hot water releases its thermal energy to the up-flowing air in the direct-contact heat and mass exchanger (DCHME), the water flow stream becomes cold water with a temperature close to the ambient temperature. The cold water exits the DCHME and enters the regenerator condenser from the top. In the regenerator condenser, the direct contact mass and heat transfer is from the exhaust air-vapor mixture to the cold-water flow stream. After recovering heat and water from the exhaust air-water mixture, the water flow stream exits the regenerator condenser at the bottom with an increased temperature and is pumped back into the storage. The benefit of this return water with an increased temperature is an increase in the overall thermal efficiency of the onboard power system. When the return water is thermally recharged by a heat source (not shown) to a high temperature such as around 300oC, the energy consumption by the heat source is reduced because the temperature of the return water is higher. Returning to the regenerator condenser, after passing water and heat from the up-flowing exhaust air- vapor mixture to the down-flowing water, the air-vapor mixture with a substantially reduced temperature and vapor content exits the regenerator condenser from the top and is directed into a radiator where the remaining vapor in the mixture is substantially condensed. Upon flowing through a water separator, the separated water is pumped back into the storage while the remaining air is discharged into the ambient. Compared to the Rankine cycle system in Fig.9, the capacity requirement of the radiator herein in Fig.10 is drastically smaller as most of the vapor in the exhaust air-vapor mixture out of the expander has been condensed and recovered in the regenerator condenser. It is important to note that, similar to the Rankine cycle–based heat engine described in this invention, hybrid vehicles combining hot-water power based on the enhanced Brayton cycle with battery-electric power may also be utilized. This configuration can be particularly beneficial for vehicle startup and stop-and-go driving in urban environments. It is well recognized that dynamic compressors and expanders, such as turbines, offer distinct advantages in terms of high power density and efficiency. In contrast, positive-displacement compressors and expanders are inherently well-suited for applications involving low to moderate power demands and frequent stop-and-go vehicle operation. Figure 11 schematically illustrates an assembly of a compressor and expander—similar to the configuration shown in Fig.10—featuring fine liquid spray or mist injection and recovery for internal compressor cooling. Referring to Fig.11, ambient air enters the compressor with a dry-air mass flow rate of ^Ỳ (its vapor content may be very low and not shown herein), and a mist injection system is employed to inject liquid oil mist into the compressor as the air coolant, which may have an inlet temperature close to the ambient temperature. During compression, a significant portion of the heat generated is absorbed by the injected mist, resulting in a much lower outlet air temperature compared to compression without mist injection. This process allows the compressor to approach isothermal operation, which greatly reduces power consumption while maintaining a low discharge air temperature. The performance of the compressor can be evaluated by isothermal efficiency, ^^&a= ^^&a / ^Yb^, wherein isothermal work consumption per unit mass ^^&amay be normally based on inlet ambient air temperature and the high-end isothermal efficiency may reach 85-90% for a rotary screw compressor. Although it may be lower than that of axial compressors (up to 95% based on isentropic work consumption), the actual compressor performance may be underappreciated, as ^^&amay be much lower than the corresponding isentropic work. Also, the internal cooling may be coupled with inter-stage cooling. The compressed air containing oil mist exits the compressor and passes into a separator, where the oil mist is extracted. The recovered oil may then be cooled to a lower temperature using an oil cooler (not shown) and reused for reinjection into the compressor. The compressed air exits the oil separator and enters a direct-contact heat and mass exchanger (DCHME), such as the one illustrated in Fig. 10, where it absorbs thermal energy and vapor from the hot water, forming an air–vapor mixture. This high-energy- content mixture then flows into the expander to generate mechanical power. In addition to its elevated energy content, the mass flow rate of the mixture entering the expander can be significantly greater thanthat of the air entering the compressor: ^Ỳ^1 + P^ vs. ^Ỳ, wherein W (humidity ratio) is the ratio ofvapor mass to dry air in the air-vapor mixture. Under certain operating conditions, the vapor mass generated within the DCHME can exceed the dry air mass (W > 1), with the vapor carrying substantially more energy per unit mass than dry air. Since the expander’s work output depends on both the mass flow rate and the specific energy content of the working fluid, this increased vapor mass—combined with its higher energy density—can significantly enhance the expander’s power output and improve the overall thermal efficiency of the heat engine, even at moderate operating temperatures. In addition to oil, the use of liquid water as an internal coolant for the compressor is also within the scope of this invention. In certain applications, a mixture of water and oil may be used—where water provides effective heat absorption due to its high specific heat capacity, and oil serves primarily for lubrication. The compressor shown in Fig. 10 or Fig. 11 may be of any suitable type, including but not limited to rotary screw, reciprocating, rolling-piston rotary, rotating-vane rotary, scroll, or dynamic compressors. Similarly, the expander can be selected from various types, such as dynamic, reciprocating, rolling-piston rotary, rotating-vane rotary, scroll, or rotary screw expanders. In typical operation, the compressor and expander function as a paired unit. One preferred configuration involves coupling a multi-stage, internally cooled rotary screw compressor—with an isothermal efficiency of approximately 85–90%—to a multi- stage radial turbine expander—with an isentropic efficiency around 90%. This pairing presents a promising solution for achieving a compact, efficient, and reliable thermal power system in the 100–500 kW range. At low operating temperatures and pressures, the Direct-Contact Heat and Mass Exchanger (DCHME) depicted in Fig.10 can resemble the packing in the wet cooling towers commonly utilized in steam power plants and HVAC evaporative coolers. However, their performance objectives differ. In the presented DCHME, the primary goal is vapor production for power generation in an expander. In contrast, wet cooling towers aim to lower cooling water temperature for use in power plant condensers or chillers. Typically, a packing (or fill media) is employed within these towers to maximize the air-water contact area, thus enhancing heat and mass transfer. At operating temperatures ranging from 240 to 300°C of this invention, however, the hot water exhibits an extremely high vapor pressure. Under these conditions, the mass transfer driving force—defined as the difference between the vapor pressure of water at its surface and the partial pressure of the vapor in the air-vapor mixture—is exceptionally large, especially when the air-vapor mixture temperature is slightly below that of the water. Consequently, any air introduced may rapidly become saturated with vapor through direct contact, resulting in a dramatically higher vapor production rate per unit volume compared to conventional wet cooling towers operating at ambient conditions. Even a compact DCHME unit can achieve substantial vapor generation capacity under these circumstances. The DCHME can be effectively operated as a Counterflow Direct-Contact Spray Tower (CDCST) without the typical packing fill media found in traditional wet cooling towers. In this configuration, hot water from the storage enters at the top, typically via spray nozzles, while cooler compressed air is introduced at the bottom. As these fluids flow counter currently, intense heat and mass transfer occurs. The water cools predominantly through evaporation, and its exit temperature at the bottom can approach the compressed air inlet temperature. Simultaneously, the compressed air gains vapor and sensible heat from the descending water, emerging at the top as a saturated air-vapor mixture at a temperature near that of the incoming hot water. To further reduce the size of the CDCST, baffles may be installed to slow down the progress of the hot water and increase the contact between the hot water droplets and the air / vapor mixture. Finally, vehicles employing the onboard hot-water thermal energy storage power systems described in this invention may offer the following additional features: 1. Hot-Water Charging: Vehicles can be quickly charged at dedicated hot-water stations, which could also be integrated into existing fueling stations if standalone setups are not preferred. Charging times could be similar to diesel refueling, approximately 10 minutes. 2. Onboard Hot-Water Storage Safety: Safety measures include using multiple smaller-diameter cylinders with integrated safety valves. In the unlikely event of a cylinder rupture, straightforward protection measures would effectively prevent personal injury. Operating at temperatures around 240°C and pressures around 33 bar further reduces risks. Importantly, onboard water storage poses no fire hazard, unlike gasoline tanks or battery packs. 3. Diverse Heat Sources for Emerging Markets: Beyond solar energy, hot water for onboard storage could be generated from geothermal energy, industrial waste heat, biofuels, hydrogen fuel, or agricultural waste. While developed regions may prioritize vehicle performance and convenience with grid electricity or petroleum fuels, rural areas in developing regions could utilize locally available resources, such as crop residues (corn, wheat, rice, cotton stalks, leaves, stems) or manure-produced methane, to produce hot water stored underground and used as fuel when needed. 4. Nature-Based Solution (NBS) for Transportation: Water is globally recognized as a fundamental natural resource. Onboard hot-water storage-based vehicles represent a genuinely sustainable, clean, and green transportation solution—thus qualifying as a Nature-Based Solution (NBS) to combat global warming. Since global warming is fundamentally nature-related, effective solutions could similarly be nature-based. Given transportation's significant share of greenhouse gas emissions, hot-water-powered vehicles could provide an ultimate pathway toward sustainability, delivering positive impacts for both the environment and transportation sectors. So far in this invention, onboard hot-water thermal energy storage systems have been disclosed. However, other onboard thermal energy storage systems employing storage media other than water are also within the scope of this invention. Said non-water onboard storage may include molten salts (like sodium and potassium nitrate mixtures), phase change materials (PCMs) suitable for high-temperature storage that might include metallic alloys or salt hydrates, and thermochemical storage using reversible chemical reactions to store and release heat, which may involve reactions like hydration / dehydration of salts, or the dissociation and recombination of chemical compounds. Fig. 12 shows schematically a thermal-energy storage system 150. The storage medium 152 may be a phase-change material (PCM), a sensible-heat-based thermal-energy storage material, or water. Although the working fluid of the thermal-energy-to-mechanical power conversion system may be air, other gaseous fluids may also be used as the working fluid in Fig. 12. The energy charge or recharge may be realized through electric heaters 154 with an electric charging cable 156. The thermal-energy-to-mechanical-power conversion system (the heat engine) may include a compressor 158, an expander 160, and a heat exchanger 163. Although any suitable type of heat exchanger may be employed and the heat exchanger may be installed entirely inside the storage system, the heat exchanger shown in FIG. 12 is a heat-pipe heat exchanger with a heat-pipe evaporator section164a being located inside of the storage system 150 while the heat-pipe condenser section164b is located outside of the storage system. The cold air or another gaseous working fluid 166a is compressed to a higher pressure by the compressor 158 and the pressurized working fluid 166b is ducted into the condenser section 164b of the heat pipe system to acquire heat from the storage medium 152 through the heat transfer from the evaporator section 164a to the condenser section 164b. The gaseous working fluid 168 with an increased temperature exits the condenser section 164b and enters the expander 160, wherein part of the thermal energy is converted into power to drive the vehicle. The expanded working fluid 170 exiting the expander may be exhausted into the ambient for an open-cycle power-conversion system or be cooled by a cooler (not shown) and returned to the compressor to complete the cycle for a closed-cycle power-conversion system (not shown). Similar to the heat engine in Fig. 10 and Fig. 11, the compressor may be internally cooled through oil or water. After the coolant 174 is separated from the working fluid 166b (separator not shown), a cooler (or radiator) 162 is employed to reduce the temperature of coolant 174 before the coolant is recirculated into the compressor 158. The gaseous working fluid shown in Fig. 12 may be substituted with a fluid capable of undergoing a phase change during operation, such as water or other suitable liquids. Aside from heat acquisition via a heat exchanger from the storage medium 152, the thermal-to-power conversion system may resemble those described in Figs. 9 or 10 and will therefore not be detailed again here. If the system mirrors the configuration in Fig.9, heat exchanger 163 in Fig.12 would operate as a steam generator, and compressor 158 would be replaced by a liquid pump, with a condenser added to handle exhaust steam (not shown in Fig. 12). Alternatively, if the system aligns with the configuration in Fig. 10, components such as a Direct-Contact Heat and Mass Exchanger (DCHME) and a regenerator condenser may be incorporated into Fig.12 (also not shown). It should be noted again that the heat-pipe heat exchanger 163 may be just one of the options. A non-heat pipe heat exchanger may be installed inside the thermal-energy storage system 150, and the working fluid of the heat engine (166b in Fig. 12) may enter the heat exchanger from the bottom of the storage system, and exit the storage system from the top, so that the heat transfer between the working fluid of the heat engine and the thermal-energy-storage medium 152 may approach counterflow configuration, which would increase the system efficiency (not shown).

Claims

CLAIMS What is claimed is:

1. An onboard thermal energy storage-powered vehicle comprising: an onboard hot-water storage unit, wherein said hot water is in at least one of the following states: liquid, liquid–vapor two-phase mixture, or vapor; an onboard thermal-to-mechanical energy heat engine system, wherein said storage unit supplies thermal energy to said heat engine, which converts some of the thermal energy into mechanical power as the primary power source to drive the vehicle without combustion; and an energy charging mechanism for said storage unit, wherein thermal energy is supplied through at least one of the following methods: charging hot water into the storage unit, or using electricity to generate thermal energy for the storage unit.

2. The vehicle according to claim 1, wherein said vehicle is at least one of the following: a passenger car, sport utility vehicle (SUV), van, bus, truck, agricultural equipment, construction equipment, mining equipment, rail transportation system, or marine vessel.

3. The vehicle according to claim 1, wherein said hot water is generated from at least one of the following energy sources: electricity, solar energy, geothermal energy, industrial waste heat, biofuels, hydrogen, agricultural crop residues, or manure management.

4. The vehicle according to claim 1, wherein said heat engine system comprises a compressor, an expander, a direct-contact heat and mass exchanger (DCHME), and a regenerator condenser; wherein hot water enters the DCHME from the top and airflow enters from the bottom, generating an air–vapor mixture through evaporation; said mixture enters the expander to produce power, and the exhaust enters the regenerator1condenser where remaining thermal energy and vapor mass are substantially recovered by cold water exiting the DCHME.

5. The vehicle according to claim 4, wherein said compressor is a positive-displacement type incorporating internal cooling via liquid mist injection and recovery; said coolant is at least one of: oil, water, or a mixture thereof; and wherein said expander is at least one of the following types: dynamic, reciprocating, rolling-piston rotary, rotating-vane rotary, rotary screw, or scroll; and wherein the mass flow rate and energy content entering the expander from the DCHME are increased due to the higher vapor content in the mixture.

6. The vehicle according to claim 4, wherein said direct-contact heat and mass exchanger (DCHME) is a direct-contact spray tower (DCST).

7. The vehicle according to claim 1, wherein said thermal energy storage unit is integrated into the vehicle without significantly increasing its width or height, with storage capacity adjusted by modifying the unit's thickness along the vehicle’s longitudinal axis.

8. The vehicle according to claim 1, wherein said vehicle is a hybrid that combines an onboard thermal energy storage power system with a battery-pack-based electric power system.

9. The vehicle according to claim 1, wherein said thermal energy storage unit is charged with hot water at a stationary charging station, and said hot water is produced using grid electricity.

10. The vehicle according to claim 1, wherein said thermal energy storage system comprises at least one of the following media: molten salts (e.g., sodium and potassium nitrate mixtures), high-temperature phase change materials (PCMs) including metallic alloys or2salt hydrates, or thermochemical storage media using reversible chemical reactions, such as salt hydration / dehydration or chemical compound dissociation / recombination.

11. The vehicle according to claim 10, wherein at least one heat exchanger is employed to transfer heat from the storage medium to the heat engine’s working fluid, including the use of heat-pipe heat exchangers.3

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