Hydrogen in the natural gas grid
This way, it is possible to use electricity effectively, especially in times when there is a significant surplus. Power plants can continue to produce electricity instead of having to shut down. However, it must be taken into account that this involves a conversion loss of about 30 %. Consequently, the first thing to check is whether the surplus electricity can be used elsewhere.
Hydrogen – the allrounder
After the production of hydrogen, there are a number of variants for its further use.
For example, it is possible to convert the surplus electricity that has been temporarily stored as hydrogen back into electricity using the reverse technology of electrolysis - the fuel cell. The electricity grid would thus experience an increase in storage capacity in addition to the existing storage power plants. However, the reconversion of electricity (fuel cell) also involves losses, which means that the chain efficiency is about 49 %. The remaining 51 % is available as heat, which could, however, be used further. (See also blog: "benefits-and-feasibility-of-storing-renewable-energies-using-hydrogen")
Hydrogen compatibility in the gas network
However, since hydrogen and natural gas have different chemical properties, a feed-in is subject to certain conditions. With regard to the natural gas grid, the components installed, the hydraulic grid capacity and the billing method must be checked in particular.
Hydrogen compatibility of the components
Regarding the components, the functionality and material compatibility of the installed system components are decisive for the concentration of hydrogen admixture. For example, the combustion properties of the gas change with increasing hydrogen concentration. Components of the gas application (e.g. forced draught burners, gas turbines), which are usually highly sensitive to the combustion properties, must therefore be tested. Particularly in the area of private applications, the range of hydrogen-capable consumer appliances is meanwhile increasing significantly.
Hydrogen changes the flow velocity in the gas network
Considering the hydraulic network capacities, it must be ensured that the connected end consumers are not undersupplied despite the admixture of hydrogen. In order that the same amount of energy can continue to be provided with an increasing hydrogen concentration, an increase in the volume flows or flow velocities prevailing in the network is necessary, which in turn has an effect on the pressure losses in the network. Since hydrogen has a lower density than natural gas, the pressure rise caused by the increase in velocity is somewhat relativised. Taking these influencing factors into account, a maximum increase in pressure losses of about 20 % is to be expected. Accordingly, it must be ensured that the distribution network has corresponding surplus hydraulic network capacities.
Changed consumption metering
The billing method must also be checked and, if necessary, revised when hydrogen is added to the natural gas grid. Both volume measurement and fuel value determination matter. The quality of the volume measurement depends on the components used and the respective measurement tolerance.
The determination of the fuel value, on the other hand, is much more difficult, as it depends on the existing network topology and the selected fuel value districts. The gas distribution network operator must always ensure that the customer is billed with the actual received fuel value. The monthly feed-in fuel value (Hs,m ) may not deviate by more than 2% from the monthly billing fuel value (Hs,m,eff ).
When hydrogen is added, a situation can quickly arise where this can no longer be guaranteed with the existing system. In that case, the customer might pay too much or too little for the energy consumed. Accordingly, suitable measures must be taken to ensure that customers in a fuel value district are properly billed. Two exemplary steps to be mentioned in this context are the introduction of gas quality tracking and the revision of the network in combination with a new division of calorific value districts.
Hydrogen in the natural gas grid as a bridge builder for a new infrastructure
Feeding hydrogen into the gas grid usually involves a great deal of effort. The components, the network capacity and the billing method must be adapted accordingly. However, the existing infrastructure already offers the possibility of adding hydrogen in small quantities (10 vol.-% H2) [1]. Currently, the injection of 30 vol.-% hydrogen is being investigated in a municipality in Baden-Württemberg [2]. In any case, the natural gas grid can serve as a bridge for the further expansion of the hydrogen infrastructure.
Combustion of hydrogen - only a transitional technology
At this point, however, we would like to point out that valuable exergy is wasted in the combustion of green hydrogen. When 100 kWh of renewable electricity is converted into hydrogen, 30 % energy is lost. Moreover, in the natural gas grid the green hydrogen is burnt . However, the combustion of 100 kWh of renewable hydrogen only results in about 70 kWh of heat instead of 400 kWh of heat if the electricity were used to operate a heat pump.
Sources:
[1] Wissenschaftlicher Dienst – Deutscher Bundestag; „Grenzwerte für Wasserstoff (H2) in der Erdgasinfrastruktur“; https://www.bundestag.de/resource/blob/646488/a89bbd41acf3b90f8a5fbfbcb8616df4/WD-8-066-19-pdf-data.pdf; 2019
[2] SWR; „Heizen mal anders: Wasserstoff im Öhringer Erdgas“; www.swr.de/swraktuell/baden-wuerttemberg/heilbronn/testversuch-mit-wasserstoff-im-erdgas-in-oehringen-100.html