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NETWORK / 22 / DECEMBER 2017 / JANUARY 2018 W ith the last coal- fired power plants in the UK mandated to close as soon as 2022, renewable sources are antici - pated to provide 34 per cent of electricity by 2020. Their inher- ent intermittency has raised the necessity to transition from large scale power production plants to decentralised and largely gas-fired on-demand production. Gas will play a central role in reshaping the energy landscape, as it is readily storable within gas networks and easily converted to meet periods of high electricity demand. Addi - tionally, there has been a rise in the adoption of residential and industrial scale CHP in a move towards local generation of heat and power. For gas distribution networks, the growing gas-electric interface and unpredictability of real-time load balancing will represent new operational stresses. One key asset impacted by growing inter-day and inter-hour load vari - ability will be pre-heat infrastructure. The gas distribution industry relies on preheating to avoid freezing. When gas is transferred from high pressure networks to lower pressure net - works, preheaters are used to prevent subse- quent freezing, overcoming a thermodynamic principle called the Joule Thompson effect. A key challenge to ensuring effective and ef- ficient use of energy in pre-heating processes is matching heat supplied with increasingly variable, unstable hourly gas demand. Current preheat infrastructure installed in the UK is one-third comprised of water bath heaters originally installed in the 1970s and 1980s, with the balance using more modern and efficient modular boiler houses. While Water Bath Heaters (WBH) have been in use on UK gas networks for more than 40 years, they waste a significant amount of energy and typically have efficiencies well-below acceptable modern standards of performance. Over the past two decades more than 500 water bath heater sites have been upgraded to more efficient Modular Boiler and Modular Condensing Boiler Houses (BH). Boiler houses also boost combustion efficiency by maintain - ing low water temperatures (below 55˚C and more preferably below 30˚C) to extract heat from exhaust gases. Both WBHs and BHs rely on the difference in temperature between warmer water and cooler natural gas to trans - fer energy. One technical challenge for systems that use liquid water as an indirect heat transfer media is achieving a controlled heat transfer to match changes in demand. While it is relatively simple to transfer energy from a concentrated heat source, such as a flame, into water with little energy loss, it is far more complex to control heat transfer in exact amounts from a dilute energy source such as warm water to process gas. The challenge is that liquid water has limited ability to absorb and transfer energy which in turn requires a relatively large mass of water to be used to ensure peak heat demands can be met. Thermal lag, or system inertia, reflects the time required for a heating system to adapt to a change in demand. Systems with high inertia are more difficult to control as the heat required o™en arrives late and when heat is no longer needed the energy already stored in the system results in a temperature over-shoot. The combination of a high-thermal inertia with lower operating temperatures lengthens the time required to transfer heat, which can lead to considerable lag in system response, as highlighted in figure 1. The figure illustrates the disturbance of a constant control temperature established overnight with a quick change in morning gas loads, followed by a temperature drop and response overshoot. The most practical way for a conventional water based technology to satisfy heat demand with rapidly chang - ing loads is to increase the margin of safety available for a sudden change in flow with a higher operating temperature (set point). In turn, slow response and overcompensation for demand periods leads to more energy being stored in the system, which ultimately results in using more fuel than necessary and over- heating. In a Strategic Heat Study undertaken by SGN & ProHeat Systems across eight UK pressure reduction stations, results illustrated consistent overheating, representing up to 30 per cent of annual fuel consumption. Network efficiency and flexibility is aligned with preheat equipment capable of match - ing energy required with energy delivered, dynamically and in real-time. Since 2014, four- teen dual-phase Immersion Tube Thermosy- phon Heaters have been installed in preheat applications across three UK gas distribution networks. These new systems use low-pres- sure steam to concentrate and control heat so that just the right amount is used exactly when it's needed. Figure 2 illustrates the energy carrying capacity of steam under a partial vacuum. Steam holds a significant amount of energy on a unit mass basis (between 2,250 and 2,400 kilojoules per kilogram). Since most of the energy content of steam is stored as GAS DISTRIBUTION Pre-heat technologies ProHeat Systems is a British design and professional engineering consultancy specialising in energy conservation for critical infrastructure. Here, Stefan Romocki looks at fast responding pre-heat technology for dynamic and responsive gas distribution networks.