The production of power from the combustion of fossil fuels contributes to global warming and air pollution. The transportation industry in Europe produced 838 million metric tonnes of CO2 in 2019. Moreover, the World Health Organization (WHO) estimates that burning carbon-heavy liquid and gaseous fuels in heat exchangers to generate energy exposes more than 80% of people living in urban areas to air quality levels exceeding WHO limits.A switch to electric or zero-emission vehicles powered by renewable energy would result in 110,000 fewer fatalities, 2.8 million fewer asthma episodes, and avoidance of 13.4 million sick days by 2050, according to recent data from the American Lung Association.

Challenges of Hydrogen as a Carbon-Free Fuel

In an internal combustion engine, hydrogen serves as an alternative carbon-free fuel, yielding nearly zero CO2 emissions (with small amounts generated through the indirect burning of lubricating oil). When used in a fuel cell to generate electricity for charging a battery that drives a traction motor, hydrogen results in zero CO2 emissions. To achieve the equivalent specific energy content of 50 liters of gasoline (typical for a medium-sized light-duty vehicle’s fuel tank), one must compress gaseous hydrogen to around 3000 bar at 25°C. To achieve the equivalent specific energy content of 50 liters of gasoline (typical for a medium-sized light-duty vehicle’s fuel tank), one must compress gaseous hydrogen in a heat exchanger to around 3000 bar at 25°C. Larger tanks of 100 and 150 liters reduce the required pressure to approximately 1500 bar and 1000 bar, respectively.
Larger fuel tanks can further reduce storage pressure, although it becomes more challenging to package them in vehicles other than heavy-duty ones. Moreover, heavy-duty vehicles will require more fuel to achieve a suitable driving range, necessitating either the expansion of tank size or the raising of pressure inside smaller tanks. Furthermore, the current hydrogen refueling infrastructure in the United Kingdom operates only 15 hydrogen fueling stations, and in Europe, there are only 155 completely functioning hydrogen filling stations, making it likewise complicated and fraught with difficulties.

PowerPaste for Hydrogen Storage and Release

While the mobility industry presents some difficult challenges for gaseous hydrogen storage, a hydrogen-rich metal hydride paste known as PowerPaste is a promising option that might gain favour for power generation. Scientists from the Fraunhofer Institute for Manufacturing Technology and Advanced Materials in Dresden, Germany, created this material. To make the paste, magnesium and hydrogen are combined to form magnesium hydride. Manufacturing is often more energy-efficient from an input standpoint than creating pure hydrogen or turning oil into liquid fuels. When utilised in equipment with a power output of 100W to 10kW, it is manufactured at a temperature of 350°C, and a pressure of 5-6bar, with very advantageous production costs.
PowerPaste is a desirable choice for light and medium transportation sectors due to its (relatively) high energy storage density (5.8MJ/kg) in comparison to the most recent generation of lithium-ion batteries (1MJ/kg), particularly if utilised as a part of a hydrogen fuel-cell or hybrid powertrain. Powerpaste is held in replaceable containers that consumers can switch out easily. It does not require any specialised equipment, resulting in shorter refuelling periods than Battery Electric Vehicles (BEVs) and less dependence on charging stations.
However, the poor absorption and desorption kinetics of hydrogen greatly compromise the storage capacity of metal hydrides used for hydrogen storage, including the magnesium-based system at the core of PowerPaste technology. Optimized heat exchangers facilitate faster heat dissipation, increasing the rate of hydrogen absorption and desorption, ultimately improving storage performance and shortening recharge times.

Optimised Heat Exchangers for Improved Performance

The key to enhancing the performance of metal hydride reactors such as PowerPaste is proper heat transmission during exothermic and endothermic processes. To control the hydrogen charging flow at the required rate with the highest storage capacity, it is necessary to remove the generated heat from the reactor. Conversely, to increase the rate at which hydrogen is released during the discharge process, one must apply some heat. Many studies have investigated design and optimisation based on various elements, including operating parameters, metal hydride structure, etc. in order to optimise the heat and mass transfer properties. Unfortunately, it is difficult to incorporate solutions to both dissipate and introduce heat without sacrificing some performance characteristics of the hydrogen-rich solution.
Optimisation of heat exchangers to improve metal hydride performance during hydrogen discharging and recharging can be classified into two types:

1.)Incorporate internal heat exchangers within the metal hydride bed.

2.) external heat exchangers that take the form of fins, cooling jackets, and water baths that cover the metal hydride bed.

Internal heat exchangers:

Improving heat and mass transfer characteristics that improve metal hydride storage performance requires increasing the heat transfer area by including fins and heat exchangers integrated into metal hydride canisters. Various internal heat exchanger designs, including straight tubes and helical coil tubes, circulate the cooling fluid in the metal hydride reactor. With an internal heat exchanger, the cooling or heating fluid will transfer local heat inside the metal hydride reactor during the hydrogen desorption and absorption processes. To boost metal hydride (PowerPaste) performance, some researchers have used several straight tubes as heat exchangers. Their findings showed that employing straight tubes as heat exchangers decreased the absorption time.
Employing straight tubes also shortened the hydrogen desorption time. The hydrogen charging and discharging rates increased as the cooling fluid flow rate rose.However, we found that increasing the number of cooling tubes had a greater positive effect than increasing the flow rate of the cooling fluid. This was likely the effect of the higher surface cooling area that is proportional to the number of cooling tubes utilised. Other researchers have investigated the performance of multi-tube heat exchangers inside the reactor by using metal hydride materials like LaMi4.7Al0.3. They suggested that the operating parameters, particularly supply pressure and the cooling fluid flow rate, substantially impact the absorption process.However, we discovered that the absorption temperature was less important.

External heat exchangers:

There has been less research on external heat exchangers to improve metal hydride absorption and desorption compared to internal heat exchangers. However, some researchers have examined the effectiveness of a metal hydride reactor for an external heat exchanger by using cooling water as a jacket to lower the temperature inside the reactor. We used a reactor with 22 circular fins and another reactor that naturally cools through convection to compare the findings. They suggested that incorporating a cooling jacket lowered the metal hydride’s temperature, improving the absorption rate. According to a numerical analysis of the metal hydride reactor with the water jacket, the hydrogen supply pressure and heat transfer fluid temperature were the main factors influencing the rates of hydrogen absorption and desorption.

Other factors:

Including high heat conductivity materials like metal foams integrated within the metal hydride bed can raise the effective thermal conductivity of a metal hydride reactor, thereby improving the hydrogen discharge process. This technique increases thermal conductivity from 0.1Wm-1K-1 up to 2Wm-1K-1.Unfortunately, adding solid material drastically decreases the storage capacity of the metal hydride reactor, making this approach counterproductive.

The Future of PowerPaste and its relevance for Mobility Applications 

PowerPaste is concentrating its growth as a fuel with a net-zero carbon footprint in the mobility market. Future studies, however, might look at the use of PowerPaste systems in stationary power generation scenarios. To ensure their appropriateness for use in the power generation sector, we will need to address several packaging concerns. Anticipate substantial growth in PowerPaste over the next ten years, primarily due to the stagnation of solid-state battery technology and its smaller size and higher energy density compared to the most recent generation of lithium-ion batteries.However, advanced heat exchangers must be adopted to improve the rates of desorption and absorption; otherwise, the implementation of PowerPaste will face some limitations.