Factsheet By-products of Electrolysis

Green hydrogen can be produced in electrolysis plants using electricity from renewable energy sources. From 2025, the first 100 MW-scale electrolysis plants are expected to begin operating; by 2030, 10 GW of electrolysis capacity is to be installed in Germany. Waste heat and oxygen are produced as by-products of the electrolysis process. To achieve the highest possible overall efficiency, it is important to determine to what extent these by-products can be utilised. This factsheet provides an overview of the topic and answers key questions. Beyond these general principles, the usability of the by-products depends particularly on the location of the electrolysis plant and must therefore be assessed on a project-specific basis.

The use of by-products can increase the efficiency of an electrolysis plant. However, this does not affect the plant’s primary purpose. The production of hydrogen will always be the primary focus in electrolysis and the central factor for the location and economic viability.

1. How much waste heat and oxygen are generated as by-products?

In a modern PEM electrolysis, approximately 75 % of the electrical energy used is converted into hydrogen; the rest is released as waste heat. In total, electrolysis with an output of 100 MW per hour produces the following by-products:

• 31 MW thermal output as waste heat
• 15 tonnes of oxygen

Waste Heat

2. How can the waste heat be utilised?

The potential for utilising waste heat depends largely on its temperature. The exact figures are determined by the type of electrolysis technology in use. For the two main technologies—alkaline electrolysis (AEL) and PEM electrolysis – the waste heat temperature typically ranges between 50 and 60°C. This is considerably lower than the temperatures found in most existing district heating networks, which usually range from 80 to 130°C.

This leads to the following utilisation options:

Direct use for low-temperature heating networks. However, these generally do not require the quantities of waste heat that are produced by large-scale electrolysis.

• Increasing the temperature level with a heat pump. The waste heat can then be fed into district heating networks. Several large-scale electrolysis plants are already planning to implement this approach. Waste heat from large data centres, which is also in the region of 50°C, is also being made usable in this way.
• Integration into existing industrial processes, provided the location of the electrolysis plant allows for it. For example, this is feasible when electrolysis plants are built close to chemical parks or refineries.
• Preheating the process water for offshore electrolysis operations.
• As heating for greenhouses – provided agricultural operations are located in the vicinity of the electrolyser.

3. What prerequisites must be met for waste heat to be utilised via a district heating network?

An electrolyser does not operate continuously. They operate primarily when there is abundant renewable electricity from wind and solar power. Current calculations assume around 4,000 fullload hours per year. A so-called intermittent heat supply must therefore be assumed. As a result, the district heating network can not rely on the electrolyser for baseload heat. If no waste heat is supplied, backup heat provision must be available, e. g. through a boiler.

The challenge is that this backup heat provision must be supplied in particularly during cold, calm high-pressure weather conditions, when little renewable electricity is available for the electrolyser. Electricity-powered backup heat provision therefore increases electricity consumption precisely when electricity is already scarce. Alternatively, hydrogen could be used in combined heat and power generation, which in turn would be a very expensive option.

Depending on the use of waste heat from electrolysis, it must be brought to the required temperature level by means of a heat pump.

4. How efficient is the utilisation of waste heat in a district heating network?

Making use of waste heat is invariably more efficient than allowing it to go unused. Assuming 4,000 full-load operating hours per year for a 100 MW electrolysis plant, this would theoretically yield a heat output of approximately 120,000 MWh. This is equivalent to the annual heat requirement of around 6,000 detached houses.

However, due to the low temperature of the waste heat, additional energy must be supplied— thereby reducing the overall efficiency of the electrolysis waste. The economic viability therefore depends on the extent of any additional investment required. The assessment of whether utilising waste heat is feasible and worthwhile must be made on a project-by-project basis.

Another advantage of utilising waste heat is the reduced demand for cooling water, as the waste heat does not need to be dissipated via a cooling system, which relies on water in many electrolysis plants. This has no adverse effect on the operation of the electrolysis plant.

5. What happens to waste heat if it is not fully utilised, and are there any associated risks?

If the waste heat cannot be utilised, it must be dissipated via a cooling system. Various technologies are available for this, which have different water and energy requirements.

In many electrolysis projects currently under development, wet recirculating cooling systems are planned. Further details can be found in the GET H2 Factsheet:

GET H2 Factsheet Water Management in Electrolysis Plants

Oxygen

6. How can the oxygen be utilised?

Today, oxygen is typically produced through air separation. It is used in a range of applications, including steel production, the chemical industry, and refineries, as well as sewage treatment plants for water purification, and as medical oxygen.

7. How efficient is the utilisation of oxygen?

Oxygen is transported either as a cryogenic liquid in vacuum-insulated tanks, in highpressure tanks, via pipelines, or bottled in compressed gas cylinders. Wherever oxygen is used today, there are established infrastructures, delivery routes, or production facilities such as air separation units. If an electrolysis plant is situated close to a oxygen-consuming business or supplier, integration into existing infrastructures is feasible.

With 4,000 full-load operating hours, a 100 MW electrolysis plant can produce around 60,000 tonnes of oxygen per year. In Germany, annual demand is approximately 8 million tonnes, of which about 2 million tonnes are transported. Oxygen pipelines already exist in Belgium, the Netherlands, northern France, and in the Ruhr area.

8. What happens to oxygen if it is not fully utilised, and are there any risks associated with this?

Oxygen that is not utilised is released to the atmosphere via the roof of the electrolysis plant or a designated vent. In principle, dispersing oxygen into the ambient air has no adverse effects on people or the environment.