To facilitate the present study, a suite of computational tools was developed for both 0-D (zero-dimensional) modelling (i.e. a ‘flat estuary/basin’ approach to barrage function) and 2-D modelling, capable of treating in detail the hydrodynamics external to the barrage structures. The latter is considered in chapter 4.
For the 0-D approach, the methodology adopted was that presented in the text ‘Tidal Power’ by Baker (1991), as the culmination of his central role in the studies conducted by Binnie & Partners for the Department of Energy through the 1980s. This text offered a speedy means of making progress on the computational components of the study whilst broader literature searches continued. The methodology leading to the Matlab based routines ‘Turgency’ and ‘Generation’ is outlined in Appendix A.2.1. Appendix A.2.2 offers reassurance that minor head losses associated with entry to, and exit from the turbine ducts (not incorporated into the computations, since these would generally be quantified only later through the detailed hydraulic design) contribute only marginally to the uncertainty range of predictions (potentially reducing energy predictions by figures in the region of 3-12%). Appendix A.2.3 provides initial independent validation testing of the software against the earlier Department of Energy commissioned studies (UKAEA, 1984) for two potential small UK barrage schemes.
Notwithstanding a ~10% disparity arising, the Turgency/Generation routines were then applied to the possible barrage schemes at the eight major estuary locations in the UK having the greatest tidal power potential, see Appendix A.2.4.
The comparison of total annual energy produced from each barrage, see Appendix A.2.4, between the Generation program and that produced from previous studies (UKAEA, 1980, 1984; DoEn, 1989) is shown in Table 2.2.1.
Table 2.2.1: Total annual output of each barrage according to Turgency/Generation routines and DoEn reports.
Turgency/Generation
(TWh) DoEn reports (TWh) Difference (%)
Solway Firth 9.81 11.99 18.2 Morecambe Bay 4.28 5.42 21.0 Mersey 1.34 1.54 13.0 Dee 1.29 1.36 5.1 Severn 11.12 15.09 26.3 Thames 1.30 1.60 18.8 Wash 3.24 4.39 26.2 Humber 1.55 1.93 19.7 Average = 18.5%
The under-prediction of energy capture from the Turgency/Generation routines, as applied herein, by an average of 18.5% arising from the validity checks is loosely consistent with the outcomes from the main body of testing (for the Dee, Mersey, Morecambe Bay and Solway Firth) presented later in Chapter 3. Whilst the Turgency (turbine characteristics) program
could be validated directly against the example given by Baker (1991), it has not been possible to conduct more in-depth validation checking of the Generation program outcomes, due to the lack of detailed information provided in the available literature on the technical details of the earlier studies.
The most conspicious cause of the discrepancy is likely to be in the treatment of the sluicing effect where Turgency/Generation has taken the actual area of the openings (sluices plus turbine ducts) as the effective area, whilst the earlier DoEn studies imposed discharge coefficients (~1.8) arising from specific design assumptions. Exploratory runs (applied as part of the studies in Chapter 3 and incorporated in Appendix A.3.1) suggest that the resulting increase in sluicing flows can explain between 5 and 10% of the under-prediction. Furthermore, and also of particular note, is the fact that all of the DoEn studies (UKAEA, 1980, 1984; DoEn, 1981, 1989) were based on a single-regulated bulb turbine adopted for the Severn study, and turbine operation characteristics were scaled from this machine. The present work using Turgency/Generation utilises a ‘hill chart’ characterising turbine performance (see Appendix A.2.1) taken from Baker (1991) and representing a double- regulated bulb turbine unit, but normalised such that the maximum machine efficiency is not disclosed. In the above validation studies using Turgency/Generation, maximum efficiencies in a range up to 97% were used in these preliminary tests, high figures being selected partially to constrain the rated head arising from the turbine/generator combinations under investigation.
Another difference in implementation is that the Turgency/Generation routines attempt only to select generator start times to achieve maximum energy capture by means of a fixed delay (after the minimum generating head is reached) over all tides in the spring to neap cycle. In contrast, the DoEn study enacted a more refined optimisation process, where the start of generation was optimised sequentially for each tide. It might, therefore, be expected that this more rigorous approach will also account for some of the remaining disparity between the approaches.
Whilst the lack of close agreement in the validation checks is unfortunate, it is considered that the Turgency/Generation routines provide a sufficiently consistent (and perhaps slightly pessimistic) energy prediction platform upon which to base the more detailed studies to be reported in Chapters 3 and 4. Furthermore, the preliminary computations discussed above can also be employed to offer an insight into the wider contribution of tidal energy to the electricity grid.
2.2.1 Conjunctive operation
One of the perceived drawbacks of using tidal energy is the lack of continuous power output and the difficulty of incorporating the electricity into the national grid. Of course, intermittency is not a problem of tidal power alone. Most renewable sources suffer from this problem, but the power output from tidal barrage schemes is much greater than from all other renewable intermittent sources. One possible way to overcome this perceived difficulty is to consider tidal barrages operating in conjunction to provide a wider power generation window. The major question that must then be answered is whether tidal barrages could be operated continuously over a 24 hour cycle to provide base load to the national grid. There are other reasons for considering conjunctive operation, not least to see how much power is achievable through renewable energy and when that power is available.
Two regimes are examined here. The first is the greatest generation window available for all barrage sites, and thus the greatest generation window available for the full daily output. The second is the greatest energy output produced from all the barrage sites, and thus a reduced generation window.
Figure 2.2.1.1 shows that there is a large variation in the energy produced from the spring tides to the neap tides. It is also clear that there is not 24-hour coverage for any of the tides
within the spring-neap cycle. Figure 2.2.1.2 shows the power output from the spring tides. It is apparent that there is a significant base load produced, of about 4GW, across 20 hours of the day. There are two peaks of power, which are the Severn, Humber and Wash barrages and the Mersey, Dee, Solway Firth and Morecambe Bay barrages.
Figure 2.2.1.1: Power output of all barrages with maximum generation window (preliminary studies).
Figure 2.2.1.2: Power output of all barrages during spring tides with maximum generation window (preliminary studies).
These pairs of schemes are almost completely out of phase and so the possibility of using flood operation at some of the barrages in order to close the gap is not available. The same bi-modal distribution is seen in the neap tides, shown in Figure 2.2.1.3, although the total power output is a lot less. Now the base load is under 1GW, although the generation window is still almost 20 hours per day. The total annual output from the 8 barrage schemes, utilising the largest generation window, is 29.4TWh.
Figure 2.2.1.3: Power output of all barrages during neap tides with maximum generation window (preliminary studies).
A similar sequence of power distribution is seen when maximum energy output is required from the barrages; see Figure 2.2.1.4. Now, however, there is less of a window over which a base load is produced. In Figure 2.2.1.5, the same bi-modal power distribution can be seen, although now clearly there is not the same coverage in terms of time. The neap tides provide the same difficulties as the largest window case. Here again, Figure 2.2.1.6, the power output is dramatically reduced, by over 50 %, from the spring tides. The total power output available from the barrage schemes, when maximising power output, is 36.1TWh.
Figure 2.2.1.4: Power output of all barrages for maximum energy generation (preliminary studies).
Figure 2.2.1.5: Power output of all barrages during spring tides for maximum energy generation (preliminary studies).
Figure 2.2.1.6: Power output of all barrages during neap tides for maximum energy generation (preliminary studies).
When considering the potential of tidal barrages, the total power available is considerable. The maximum annual energy output represents close to 10% of the total annual electricity consumption within the UK. At present, only 4.5% of electricity generation is through renewable sources, and the government has a binding target of 20% by 2020. If tidal barrages were utilised across the above 8 sites alone, then over half of the target, under current usage, would be met. This does not take into account the drive to reduce electricity demand over the coming years, which would uplift the percentage contributions.
3. ENERGY FROM MAJOR BARRAGE SCHEMES (0-D MODELLING)