Integrated Modeling and Assessment of the Operational Impact of Power-to-Gas (P2G) on Electrical and Gas Transmission Networks
Summary (5 min read)
Introduction
- The electricity for the P2G process could, for instance, come from renewable energy which would otherwise be curtailed due to system or line constraints.
- Integrated energy systems; Multi-energy systems; Hydrogen production.
- This paragraph of the first footnote will contain the date on which you submitted your paper for review.
- On the above premises, the aim of this paper is to model and assess the possibility of integrating the P2G process into an existing energy system, with focus on modelling the impact on the gas and electrical transmission infrastructure.
- All these are unique and novel contributions to understanding the implications of P2G considering realistic network operation and constraints.
A. Operation and location of the power-to-gas facilities
- Different types of P2G facilities can be considered which depend on the end-use of the gas produced as well as its locality and reason for the curtailment of the renewable sources.
- On the other hand, SNG facilities can be placed away from gas terminals and their utilization as a means of relieving gas network congestion by altering the gas flows will also be assessed as a potential application.
- Besides for system stability requirements, curtailment may also occur due to transmission line constraints.
- When this is away from gas terminals, an SNG facility will be considered in the scenarios analyzed; on the other hand, if the curtailment occurs at a gas terminal, then both H2 and SNG will be considered.
- In the modelling and the studies carried out below, it is assumed that the locations and sizes of the three sets of P2G facilities are assigned based on prior electrical and gas network analysis that identify the relevant requirements.
B. Power-to-gas processes
- In the first type of P2G process, gaseous hydrogen is formed by the process of electrolysis whereby water is split into hydrogen and oxygen.
- Facilities of this type are currently employed in P2G for gas distribution networks [17] [18].
- The methane forming process, methanation, is a secondary process which requires H2 resulting from electrolysis along with carbon dioxide: CO2 + 4H2 → CH4 + 2H2O.
- This process may be either chemical or biological [9].
- At a system level the overall process is similar and, for the purposes of this work, no distinction will be made between the two.
C. Modelling of the injection of H2 in the gas network
- There are technical and legislative restrictions on the quantity of H2 that may be blended into the NG network.
- The legislative limits vary widely for different regions and gas networks [19].
- The amount of gas entering the network is considered in terms of its energy content.
- Since the HHV of H2, by volume, is approximately a third of that of NG, the H2-NG mix will have a smaller HHV than NG.
- This HHV value will be used to convert energy gas demand into volumetric gas demand.
D. Modelling of the CO2 emission reduction
- The P2G process, as well as offering cost benefits while it uses otherwise unutilized energy, also offers benefits in the reduction of the system carbon emissions.
- In fact, combustion of H2 does not produce any of the greenhouse gases associated with the use of fossil fuels.
- The CO2 benefits from the production of SNG are taken as the quantity of CO2 removed from the atmosphere.
A. Overall network modelling and simulation methodology
- The overall integrated network modelling process is formed of fours steps (Fig. 1), namely, a two-stage DC OPF alternated with gas network transient analysis.
- Using the gas demand requirements for electrical generation, a transient gas flow is conducted to determine the modelling parameters of the P2G operation.
- These are used in the second OPF (Section III.C) which determines the level of power injected into P2G facilities to maximize renewable integration while taking into account the renewable curtailment and the location and type of P2G facilities.
- The process is then repeated for the next time interval and so on.
- The integrated network model has been implemented and solved in MATLAB [22].
B. First stage OPF
- Starting from classical DC OPF formulation [24], the first stage OPF (Step 1 in Fig. 1) determines the dispatch 𝑃CG𝑖 1 of each generating unit CG𝑖.
- Run transient gas flow to assess impact of P2G on the gas network operation Step 3.
- Each of the renewable generators, RG𝑟, are assumed at zero marginal cost 𝑐RG𝑟 in the rest of the paper.
- 𝐽𝑙 are observed by limiting the magnitude of the real power flows 𝑌𝐽𝑙 1(𝑡) in (8).
- Reserve is fulfilled by conventional generation and characterized by the generator’s upward ramp capability 𝑅CG𝑖(𝑡), which is determined by the generator’s maximal ramp 𝑅CG𝑖 and the availability of upward generation, as in (13).
C. Gas network transient analysis and second stage OPF
- More specifically, results from the first OPF are used to determine the level and location of the curtailed wind.
- This allows for power from the renewable generation sources to be transported to the areas of gas network congestion, instead of being allocated to nearby P2G facilities.
- The results of the preliminary OPF define the generation levels 𝑃CG𝑖 1 (𝑡) and 𝑃RG𝑟 1 (𝑡) in (20).
- The power flows 𝑌𝐽𝑙(𝑡) along the lines must continue to satisfy the line constraints 𝑌𝐽𝑙 as in (21).
D. P2G operation modelling
- As mentioned in Section II.A, there are three sets of P2G facilities that have been considered in the model: P2G facilities at congested electrical nodes: this first set is composed of P2G units P2G𝑧 that could generate SNG from curtailed wind due to electrical line constraints.
- When there is low pressure and curtailed wind, a P2G facility may introduce gas into the network to alleviate the congestion.
- It is assumed that this rule as to the operational conditions for congestion relief as well as suitable facilities’ placement are obtained from prior gas network operational analysis.
- While for a H2 facility P2G𝑇𝑘,H2 with associated H2 storage of capacity 𝐶𝑇𝑘, the quantity of H2 which may be injected depends on the current level of storage 𝑉𝑇𝑘(𝑡), which defines its spare capacity 𝐶𝑇𝑘 − 𝑉𝑇𝑘(𝑡).
E. Gas network transient flow analysis model
- Gas network studies at Step 2 and Step 4 of Fig. 1 are conducted via a transient gas flow analysis model.
- Transient gas flow in a section of pipeline is characterized by three relations, namely, the equation of state and the continuity and motion equations (see equations (34), (35) and (36).
- The point 𝑘 may include a terminal or storage facility, 𝑇𝑘 , with supply/injection rate 𝑄𝑇𝑘, a gas demand 𝐺𝐷𝑘, a compressor station with inlet flow 𝐶𝐼𝑘 or outlet flow 𝐶𝑂𝑘, or a point of P2G injection 𝑄𝑃2𝐺,𝑘 1 .These are combined with the pipe flows so that if at node 𝑘 there are 𝑁𝑘 adjacent pipe sections.
- As a further study element, the power requirements of the compressor stations to overcome transportation pressure drops are modelled as in [26].
A. Case study description
- The model developed has been applied to the GB gas and electricity transmission networks in five case studies: - Case 1. P2G operational cost and environmental benefits.
- Before analyzing the case study results below, there is a description of the electrical, gas, and P2G facility data used.
B. Electrical network data
- The installed generation are those predicted by National Grid’s ‘Gone Green’ scenario in 2030 [16], where wind generation accounts for 40% (48GW) of the total installed capacity of 120GW, and there might therefore be large amounts of curtailment (peak demand is 63GW [16]).
- A A B C E F l (a) (b) St. Fergus D IEEE Transactions on Sustainable Energy – 2015 7 TABLE I GENERATION TECHNOLOGY AND RESPECTIVE INSTALLED CAPACITY, COST, MSG AND 30-MIN RAMP RATES [28].
- The system reserve considers the capacity of the largest generator, taken as 𝑅Gen = 1.8 GW for 2030 [29], plus reserves for uncertainty in load and wind generation forecast, as in [25].
- The resulting levels of wind generation and wind curtailment over a month as from the 30-min OPF are shown in Fig.
C. Gas network data
- A simplified version of the GB Gas National Transmission System with 79 nodes has been used to conduct the analysis of the gas flows and pressures across the network, as shown in Fig. 2(b).
- All gas terminals and compressor stations have been preserved.
- In fact, the flow characteristics of the pipe define the relation between the flows and pressures in the network.
- The ability of this simplified network to realistically model the pipe flows, pressures and linepack has been verified by comparison with historical gas flows provided by National Grid Gas.
- Historical daily gas demands from December 2012 have been used for network offtakes excluding the large industrial and interconnector demands, taken as 7.8GW and 5.0GW, respectively.
D. Power-to-gas facilities data
- The following P2G facilities have been considered.
- The efficiencies for the P2G producing process is taken as 73% for H2 production and 64% for SNG production [32].
- These efficiencies also include the energy required to compress the gas to 80bar, a pressure suitable for the gases’ consequent introduction into the gas network.
- This level is under review with industry petitioning for it to be raised to 3%vol. [19].
- System gas demand for non-power and power generation.
E. Case 1. P2G operational cost and environmental benefits
- The cost benefit of the P2G process, for each half-hour, is shown in Fig.
- The systems emission reduction is measured at the time at which the H2 or SNG enters the gas network (also shown in Fig. 5).
- The total CO2 emission reduction for the month is 250 kilotonnes.
- Fig. 5. System cost benefit and emission reduction from power-to-gas.
F. Case 2. Benefits of using hydrogen storage facilities
- When the level of wind curtailment is greater than that sufficient to produce the maximum levels of H2 permissible to be blended into the gas network, H2 may be put into storage, if available.
- For illustrative purposes, wind curtailment is compared against that required to meet the maximal level of H2 content of the gas network for an average winter day.
- This is shown in Fig. 6 where it can be seen that, without H2 storage, there is a large unutilized H2 content capacity of the gas network as well as curtailed wind which could have been converted into H2 for successive injection into the gas network.
- Therefore, storage facilities could allow for greater and safe use of the H2 capacity of the gas network.
- The energy saved by the introduction of storage facilities, over the monthly time frame considered, is 18GWh, corresponding to £430,000 at the considered gas price.
G. Case 3. Effects of P2G on the gas network
- The P2G process will have a number of effects on the gas network.
- The addition of large SNG facilities away from terminals will alter the network flow patterns while the introduction of H2 will reduce the HHV of the gas and increase the volume of gas necessary to satisfy the demand.
- The energy transportation capability of the GB gas transmission network far exceeds that of the installed wind generation.
- And, if the terminals are used for the large scale P2G facilities, then the inputted H2 and SNG act to displace NG supplied through the terminals.
- There is minimal effect on the gas network flow characteristics.
H. Case 4. Use of P2G to reduce electrical congestions
- Line constraints due to excessive wind generation occur on the electrical transmission line a (Fig. 2(a)).
- The P2G process can be used to relieve these line constraints by increasing the load at certain nodes by means of a P2G facility.
- Again, the value of the SNG produced can be considered based on the gas price.
- Fig. 8. SNG production following electrical transmission line constraints.
- IEEE Transactions on Sustainable Energy – 2015 9.
I. Case 5. Use of P2G to reduce gas congestions
- The P2G concept can be used to reduce congestion in the gas network by introducing SNG production units at vulnerable areas in the gas network, for example, at network extremities, where the pressures will be least.
- An alternative to pipeline reinforcements may be the installation of a P2G facility at a nearby location which would be able to inject gas into the network at times when flow problems may occur, for example, due to high demand or network failures.
- Each scenario is modelled with the P2G installations of Section IV.D with the first scenario also including the additional 2GW SNG facility at node B. Fig.
- More specifically, when there is spare electrical generation and transmission capacity then this can be used to transport energy and fulfil gas demand, whilst previously this energy would have to be transported via the gas network and would have contributed to compression costs.
V. CONCLUDING REMARKS
- IEEE Transactions on Sustainable Energy – 2015 10 operational implications that P2G programs could have, including impact on both transmission networks.
- The benefits of including H2 storage facilities as a means of capturing the curtailed winds spikes in the P2G process and reducing conversion losses have also been quantified, together with potential use of P2G facilities as a substitute to electrical and gas transmission line reinforcement.
- It has also been shown how strategically placed SNG facilities can be used as an alternative to compressor usage.
- Work in progress aims at performing cost benefit analyses that take into account planning aspects of P2G as a measure to increase system flexibility and a substitute for network reinforcement.
- In addition, a full multi-energy system [34] model that integrates the electricity, heat and gas systems is under development.
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...In [181], the introduction of P2G with an equivalent capacity of one third of the total installed capacity led to a reduction of 3–8% of the seasonal storage, given that part of the gas demand is covered by P2G....
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...In [181], the focus is on operational costs rather than total (considering investment), but these are reduced by 4–9% depending on the level of penetration (15– 30%)....
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...There is limited insight on the competition with hydrogen and its use for either mobility or injection in the gas grid ([181] explores injection, but does not include mobility)....
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...PtG is a promising technology that can contribute to tackling the issues of increased renewable generations [2], [3], [19]....
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...In addition, PtG as an emerging technology can effectively convert excessive electricity to compatible natural gas, which can be used by gas-fired units and other gas consumers [2], [3]....
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...In recent years, the emerging power-to-gas (P2G) technologies enable the reverse direction of energy conversion from electricity to natural gas [20], [21]....
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"Integrated Modeling and Assessment ..." refers methods in this paper
...The OPFs have been implemented with the support of MATPOWER [23]....
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"Integrated Modeling and Assessment ..." refers background in this paper
...Facilities of this type are currently employed in P2G for gas distribution networks [17], [18]....
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