A long time ago, when coal was still the top fuel and renewable energies were still long from being discussed, Jules Vernes made a prediction in his 1874 novel “The Mysterious Island”: “Water is the coal of the future. The energy of tomorrow is water, broken down using electrical energy. The broken-down elements, hydrogen and oxygen, will ensure the earth’s power supply for an indefinite period of time.” Almost 150 years after the French author wrote these words, his vision is still a long way from being achieved. However, given the challenges presented by climate change and the search for new energy sources, hydrogen, as an energy supply, is gaining increased attention from the world of politics, science and industry. After all, the benefits are clear: when it is burned, only water is emitted, rather than harmful carbon dioxide.
Using hydrogen in heating systems can significantly contribute towards climate protection.
The downside is that hydrogen almost always appears on earth as a compound. Obtaining it requires a lot of energy. As Jules Vernes outlined, this can be done using electrolysis, allowing water to be broken down into hydrogen and oxygen using electricity. This means that the fuel can only be climate-neutral if the electricity used comes from climate-friendly sources. This is where renewable energies come in.
Developing them will make it ever more necessary to store excess energy that cannot be fed immediately into the grid over longer periods of time. After all, the wind and sun do not answer to levels of demand. This climate-friendly electricity can be used in power-to-gas plants to produce hydrogen, for example. As households produce a large proportion of CO2 emissions, using hydrogen in heating systems could significantly contribute towards climate protection. Producing it is still very costly, but the heating technology sector is raring to go.
Hydrogen is gaining ground
Enriching natural gas with four to six percent hydrogen is already permitted, depending on the gas characteristic values as per process sheet G260 from DVGW (the German association for gas and water). But would it be possible to switch entirely to hydrogen? Several projects are under way to answer just that. For example, the British government has launched a program called “Hy4Heat” to determine which technical and logistical hurdles need to be overcome in order to gradually increasing the proportion of hydrogen. Doing so would have a particularly positive effect, considering that 80 percent of households in Great Britain rely on gas, which is a particularly high amount when compared with the international average.
The country’s third largest city Leeds is planning to move part of its gas network to 100 percent hydrogen in the medium term. Similar ideas can be found on the continent, or more precisely in the Netherlands. Here, resources from natural gas fields will soon be depleted. In Rozenburg, Rotterdam there are already field tests in place with 100 percent hydrogen enrichment. Yet, when it comes to power-to-gas plants, Germany is the country leading the way again. In test plants, researchers are investigating how to generate as much hydrogen as possible with the least amount of energy usage, and there are various studies on converting gas pipe lines into hydrogen pipes lines. In a nutshell: the market is on the move.
Challenges for manufacturers
That is why several manufacturers are working on preparing their condensing units for clean energy supplies (Fig. 1). The aim is to achieve this with as few technical and constructional changes as possible. The good news is that the current operating principle can remain the same. However, due to the characteristics of hydrogen, several issues have been to considered: leak requirements, the compatibility of materials and, most importantly, its flammability.
Taking leaks into account, hydrogen is the lightest of all chemical elements with the lowest density. It has a higher permeability than natural gas through elastomers and plastics and, due to its slightly lower dynamic viscosity, it has a slightly higher leak rate than natural gas. The tightness of the components in the fuel condensing boiler must be adapted accordingly and monitored with relevant checks. The compatibility of the materials must also be checked, and the flammability requires particular attention (see table). The flame speed is eight times higher than with methane, meaning that manufacturers cannot work with the same burners.
The pressure loss increases and the power of the blower has to be optimized. Mostly importantly, it has to be ensured that the ignition does not happen too late. Hydrogen is very reactive and ignites a lot better than methane. Therefore, the combustion controller has to factor in shorter ignition times. Another sticking point is that the usual flame measurement procedures using ionization cannot be implemented when checking and monitoring the combustion. As a result, heater manufacturers have to test new sensors or thermocouples.
Another key issue is that hydrogen has a smaller lower heating value than methane but its Wobbe index, important for the interchangeability of fuel gas, is almost as high. For an optimum mixture in the venturi, the gas/air ratio controls have to be adapted accordingly. As a result, the interaction between the gas blowers, venturi and gas valve is of great importance.
NRV 118 ready for H2
The NRV 118 (Fig. 2) from ebm-papst is already designed for use with a hydrogen proportion of up to 10 percent without changes being required. The engineers at the Landshut site put the established NRV 118 composite system through several investigations and initial field tests to check its suitability for hydrogen. The result: with some changes, the composite system can be adapted for use with 100 percent hydrogen. The tightness of the gas valve and blower was increased and the plastics and metals used were checked for their suitability.
Another benefit became clear: thanks to special pre-mixture equipment, the NRV 118 is ideally suited to hydrogen use. The “pre-fan-mix” gas blower compensates for the lower Wobbe index and hydrogen’s smaller lower heating value in the suction operation. There is no need for more complex control lines. Furthermore, higher modulations can be run because the gas valve can be optimally controlled by negative pressure.
All in all, this means that manufacturers can count on the “NRV 118 Hydrogen” when using 100 percent hydrogen. It will be some time before the clean fuel can be used extensively for heating (Fig. 3). Yet, if developments in science, politics and industry continue as they are, Jules Vernes’ fictional vision may become a reality before long.
What is important for manufacturers of fuel condensing boilers when converting to hydrogen.
- The flame speed is eight times higher than with methane. The blowers have to be adapted to the burners’ higher pressure loss.
- Due to the slightly decreased dynamic viscosity, leaks have to be considered.
- Hydrogen has a high permeability. Suitable elastomer tools have to be used.
- Flame measurement procedures using ionization are not possible with 100 percent hydrogen. Manufacturers have to test new sensors or thermocouples.
- Ignition loads and ignition times have to be kept as low and short as possible with pure hydrogen.
- The lower heating value of hydrogen is smaller than methane. This makes a perfectly coordinated gas/air ratio control even more important.