SOURCE-TO-SINK EFFICIENCY OF DISTRICT HEATING AND HYDROGEN FOR BUILDING HEAT SUPPLY

Hydrogen is commonly mentioned as a future-proof energy carrier capable of decarbonizing the future energy system. While in principle this is correct, in practice, it has a major efficiency drawback. Due to the energy intensity of its manufacturing process, it needs to be applied in a sensible way and focused on hard-to-decarbonize sectors. For building thermal demands existing solutions like district heating for urban areas and heat pumps for rural areas are more energy-efficient applications.

This article compares district heating and hydrogen-based heat supply systems for urban areas driven by natural gas and renewable energy. The results show that district heating is significantly more energy efficient than hydrogen-based heat supply; consequently, it has a much lower environmental footprint.

By Dr. Oddgeir Gudmundsson, Danfoss A/S – Danfoss Climate Solutions – District Energy, Building and Leanheat – Application Center – Projects

Introduction

To fulfill climate goals, it is necessary to decarbonize the energy system. In principle, there are many possible paths toward achieving carbon neutrality. However, different approaches have different costs, environmental footprint, and primary energy efficiency. These parameters are generally linked to the energy efficiency of the applied supply system, e.g., the higher the system efficiency, the lower the system cost, environmental footprint, and primary energy need become.

The key to achieving high energy efficiency is to minimize the number of energy conversion processes and match the supplied energy to the demanded energy quality. This is particularly important in relation to building heating demands, which are of low energy quality nature.

The Hydrogen Council, a lobby organization for the major oil and gas producers, promotes hydrogen as a viable and cost-effective way to decarbonize the heat supply in buildings currently heated by natural gas. The council encourages the idea of repurposing the existing natural gas infrastructure and avoiding developing new infrastructures. This idea is shared in a number of reports focusing on various countries, including the Netherlands, Germany, the United Kingdom’s governmental hydrogen strategy, and Europe.

While the idea of repurposing existing natural gas grids is appealing, research has shown that most of the components in existing natural gas grids are unable to cope with a large concentration of hydrogen in the heat supply. In fact, the maximum allowable blending in Europe in 2020 was in France, 6%. Due to this incompatibility, an extensive renovation of the existing natural gas infrastructure, from the transmission lines to and including the end-users gas installations, would be needed to enable the grand roll-out of hydrogen.

An alternative to hydrogen-based heat supply in urban areas could be modern low-temperature district heating (DH), an infrastructure for distributing centrally produced heat, at one or more locations, via a pipe network to heat consumers in urban areas.

This article compares blue and green hydrogen and DH as heat supply systems for building heating demands regarding energy intensity and global warming potential (GWP). As blue hydrogen is promoted as transitional hydrogen by the hydrogen industry, the article considers blue DH, an NG-based DH, as a transitional alternative.

This entire energy chain comparison of these energy carriers highlights the large inefficiencies associated with manufacturing high-quality energy carriers for low-quality energy demands. As both supply solutions aim at solving a basic need, building heat demands, and both require extensive infrastructures to be built, an inherent long-term lock-in effect must be considered.

Due to the lock-in effect, it is particularly important to prioritize energy efficiency to minimize the environmental footprint and cost of establishing the future renewable energy generation system.

Methodology

This article aims to evaluate the energy intensity and GWP of a future energy system that utilizes either blue energy, decarbonized natural gas, or renewable energy for fulfilling building heating demands. The analysis is based on examining the energy supply chain, from primary energy input to the energy system to the end-user of heat. For the blue energy systems, the system boundary extends from the gas field development to the heat-consuming buildings, see Figure 1.

Figure 1

In renewable energy systems, the system boundary extends from the point where the renewable power enters the power transmission grid to the heat-consuming buildings, see Figure 2.

Figure 2

When estimating the overall supply system efficiency, the efficiency and fugitive emission of each step is estimated, based on a literature review.

Results

While several different thermal sources would generally supply DH, this analysis assumes that both heat supply systems are based on the same primary energy input. This simplification enables a one-to-one comparison of the primary energy demands of DH and hydrogen-based heat supply systems.

The below Sankey diagrams visualize the main results of the study, the primary energy demands, the energy efficiencies along the supply chain, and the value of matching the supplied energy quality with the demanded energy quality.

Figure 3

Comparison of the Sankey diagrams for the blue scenarios, Figure 3 and Figure 4, show that the superiority of blue DH originates from the ability to capture waste heat from primary fuel conversions and by incorporating ambient heat, via heat pumps, into the heat supply.

Figure 4

Utilizing the waste and ambient heat greatly reduces the primary energy demand compared to the blue hydrogen alternative. The lower primary energy demand further leads to significantly lower short- and long-term GWP potential compared to the blue hydrogen alternative, as shown in Table 1.

Table 1

In the green scenarios, Figure 5 and Figure 6, the ability of DH to utilize heat pumps becomes even more advantageous, leading to significantly lower primary energy demand compared to the green hydrogen alternative.

For further information, please contact Oddgeir Gudmundsson, og@danfoss.com

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