COUPLING OF THE TRANSPORT SECTOR IN A 100% RENEWABLE ENERGY SYSTEM The last step here is to couple the transport sector with the other sectors. This is achievable in a smart energy system, illustrated in Figure 4. This includes direct electrification (plug-in electric vehicles or electrified rail) of vehicles and fuel synthesis for production of liquid or gaseous fuel for combustion engines. The direct electrification of transport is important to expand as much as possible because this has a much higher energy efficiency than transport based on combustion engines. Not all transport can be covered with direct electrification though, so aviation, heavy ship and land freight may also in the future need some fuel. This need for fuel may be covered with electrofuels, which is a synthesis of 1) a carbon source, such as gasified biomass or CO2 emissions from CHP plants and 2) hydrogen from electrolysis using fluctuating electricity as the energy source. The fuel synthesis is not yet a well proven technology in large scale, so there are some uncertainties related to this still. However, it looks like a promising alternative for the transport sector to integrate fluctuating renewables, up to around 75% of the total electricity demand. In this system, it is possible to cover the demands with little fuel, and potentially cover all demands with 100% renewable energy within sustainable limits of bioenergy. SUMMARY AND CONCLUSIONS The three steps, from the traditional to the smart energy system, are able to reduce the need for fuel from 133 units to only 39 units, integrating additional 39 units of wind, in this particular case covering 100 units of demand. This can be achieved through an effort to couple the different energy sectors using flexible and efficient conversion units. The transition illustrated through the figures requires large changes, not only on the technological level but also in the regulatory frameworks and organisation of the energy sector to support such a transition. The core of this is that the current organisations and institutions are designed for (and by) a centralised energy system based on fuel consumption, whereas a smart energy system is much more decentralised in its components and organisations and its economy is based more on investments rather than consumption of fuel. ACKNOWLEDGEMENTS The work presented in this article is a result of the research activities carried out in the EU Horizon 2020 project Heat Roadmap Europe 4 (www.heatroadmap.eu) and the Strategic Research Centre for 4th Generation District Heating (www.4dh.dk), which has received funding fromThe Innovation Fund Denmark.
FIRST STEPS TOWARDS SECTOR COUPLING Traditionally, the different energy consuming sectors have been completely isolated from each other, which has led to the traditional energy system structure (see Figure 1). Here, fuel is used to cover all energy demands through single-sector conversion units, and the system consumes 133 units of fuel to cover the total demand of 100 units. The power plants producing the electricity may have a high efficiency when only looking at the electricity sector, but when considering that the excess heat from the process could have been used for other purposes, reducing the consumption of fuel elsewhere in the system, the efficiency doesn’t seem that high any more.
SOME FLUCTUATION ELECTRICITY FROM WIND AND SOLAR
Figure 2 shows a system where combined heat and power (CHP) is implemented to improve the overall system efficiency through supplying the excess heat from the power production to cover heat demands. In some cases, this excess heat can cover heat demands in industrial processes, but in many cases, it will also require a district heating (DH) network to be in place. In systems with DH networks and CHP units, an investment in a relatively cheap thermal storage can support integration of fluctuating electricity up to about 25% of the demand in a cost- effective way. The thermal storage provides a flexibility for the CHP units to operate more independent of the heat demand, and more according to electricity demands, thereby being able to balance out some of the fluctuations from wind, solar PV, etc. INCREASING ELECTRICITY PRODUCTION FROM FLUCTUATING SOURCES REQUIRES NEW DEMANDS To reduce the fuel consumption further, integration of more fluctuation renewables is needed. However, this can be hard to do in a feasible way with only the conventional electricity demands. Therefore, an option can be to introduce electricity- to-heat through heat pumps or electric boilers for production of heat (see Figure 3). This can be relevant in buildings with individual heating systems, but particularly in areas covered with DH. In DH networks, heat pumps can be operated more flexibly, for example by turning off the heat pump in times with very low production of wind power and instead supplying the heat from a centralised thermal storage. DH also enables utilisation of large- scale low temperature heat sources that would otherwise be lost, e.g. excess from industry, waste water treatment or hospitals. In individual buildings, the potential for operating a heat pump in a flexible way is lower because it has to deliver the heat when needed. But a heat pump is still more efficient than a fuel-based heat-only boiler if there is no DH network available. Through all the steps, the power and CHP plants are producing gradually less electricity. In the transformation, their role is changing from being simply to deliver electricity to the electricity system to balancing the fluctuations of the renewable production. This will increase the requirements for the power and CHP plant’s ability to regulate their production up and down.
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