INTRODUCTION

  1.  Research carried out at the Environmental Change Institute at the University of Oxford has demonstrated that a mix of renewable technologies, together with a planned approach to the development of renewables across the country, has the potential to reduce significantly the overall intermittency from grid connected intermittent renewables.

  2.  This submission demonstrates that strategies to better match electricity generated by intermittent renewables with current electricity demand patterns will substantially reduce (but not eliminate) the need for stand-by generating capacity. There exists the opportunity to manage intermittency from renewables through appropriate planning, resulting in cost savings for the electricity supply industry.

  3.  This submission concerns the necessity for, scale of, and opportunities to better manage stand-by capacity requirements for large scale adoption of intermittent sources of renewable electricity generation. Intermittency is a real concern for electricity networks with large amounts of renewable generation, however the opportunity to manage this intermittency, and the cost benefits that may arise from appropriate planning, are often overlooked.

INTERMITTENCY RESEARCH

  4.  The research considered the intermittency properties of offshore wind, solar photovoltaic (solar PV) and domestic combined heat and power (dCHP)[1] electricity production, and examined strategies to reduce the overall intermittency from these technologies (onshore wind, wave and other intermittent renewables have not been considered in this research).

  5.  Using long-term environmental records, the potential hourly electricity output from offshore wind farms in four main areas around the English and Welsh coasts, together with the potential hourly electricity output from solar PV and dCHP systems in London was modelled. These modelled electricity supply profiles were then compared against the hourly electricity demand profile for England and Wales.

  6.  The computer model allowed different weightings to be attached to the intermittent electricity output from different wind, solar PV and dCHP sources—the purpose of this was to determine the mix of generating capacity from the different technologies, and in the case of wind from the different wind farm sites, that would result in the closest match between the intermittent electricity supply profile from these sites and the known electricity demand profile.

  7.  Once the optimum distribution of intermittent generating capacity amongst the wind, solar PV and dCHP sites had been determined, the research identified the system benefits of this approach, including the impact these results could have on the size of stand-by generating capacity required by the network.

  8.  A primary benefit of this finding is that the management of intermittency will result in large reductions in the stand-by capacity. Additional benefits to the electricity generation and distribution system were also identified.

COMPLEMENTARY INTERMITTENCY PATTERNS

  9.  Different patterns of intermittency are observed from different technologies, and from the same technology used at different sites. Once these differences in intermittency patterns are known, it is possible to identify complementary patterns between different sites or technologies, and thus plan the development of intermittent renewables to better match the electricity demand pattern.

Monthly Intermittent Renewable Generation

  10.  Figure 1 demonstrates how the electricity generated from a combination of intermittent technologies can match demand on a monthly basis. In this example, the total hourly electricity demand for each month has been calculated (black line), while the total electricity generated by wind, solar PV and dCHP for each month has also been calculated.

  Figure 1—Comparison of Electricity Demand and Intermittent Electricity Supply by Month, England and Wales.

  11.  In this case, the total amount of electricity generated from intermittent renewables equals 10 per cent of the total annual electricity demand for England and Wales—for each month, however, the contribution from intermittent renewables may be slightly higher or lower than the 10 per cent annual average target. Electricity generated from offshore wind accounts for around 65 per cent of total intermittent generation[2], while dCHP accounts for around 25 per cent and solar PV accounts for around 10 per cent of total intermittent generation.

  12.  An important feature of Figure 1 is that electricity generation from intermittent renewables tends to be higher during months of highest demand (winter months) and lower during months of lower demand (summer months). This feature of the intermittent supply is beneficial to meeting overall electricity demand, as intermittent renewables will (on average) contribute most when demand is at its highest.

  13.  Figure 1 clearly shows the interaction of different intermittent electricity supply types at the monthly time scale. Yet at this monthly timescale much of the intermittency inherent in these generating systems is hidden as it occurs at a much higher frequency.

Hourly Intermittent Renewable Generation

  14.  Figures 2 and 3 show the contribution of wind, dCHP and solar PV to meeting demand at an hourly resolution during one week in January and one week in July, 1999. Again, the contribution of these intermittent renewables accounts for 10 per cent of total annual electricity demand in England and Wales, however the actual contribution of these renewable technologies in any one hour varies by time and by technology.

  15.  Under both summer and winter conditions, this combination of intermittent renewable technologies contributes to meeting demand. As suggested from Figure 1, however, the relative importance of the different intermittent generating technologies in meeting demand varies between the seasons.

  Figure 2—Contribution of Conventional and Intermittent Renewable Electricity Supply Meeting Demand—6-12 January 1999, England and Wales.

  16.  An important finding from this work is that, on average, this combination of intermittent renewables contributes more to meeting peak daytime electricity demand than it does to meeting off-peak demand at other times. Indeed, while the intermittent renewables are sized to meet 10 per cent of annual electricity demand for England and Wales, on average they meet around 11.2 per cent (4.2GW) of peak demand (8 am-8 pm), and around 8.8 per cent (2.7GW) of off-peak demand (8 pm-8 am).

  Figure 3—Contribution of Conventional and Intermittent Renewable Electricity Supply Meeting Demand—6-12 July 1999, England and Wales.

  17.  It should also be noted that while the hour-to-hour contribution of intermittent renewables varies, this combination of wind, solar PV and dCHP generating capacity contributed to meeting electricity demand during every single hour during the period 1980-2000.

IMPACT ON STAND-BY CAPACITY REQUIREMENT

  18.  Figures 1-3 show how using a combination of intermittent renewable technologies in the right proportions can improve the match between intermittent supply and demand patterns. They also show that, on average, renewable electricity generated by these renewables provided a higher contribution to meeting peak loads than to meeting other loads.

  19.  However, when considering the impact of intermittent renewables on stand-by capacity requirements it is the actual hourly contribution of renewables, rather than the average contribution, that is of key importance.

What is Stand-by Capacity

  20.  It is important to define precisely what the stand-by capacity represents. When electricity generated by intermittent renewables replaces an equivalent amount of electricity generated by conventional capacity, stand-by capacity in addition to that already on the system may be required to meet demand when the intermittent renewable electricity supply is low and demand is high.

  21.  It is important to recognise that additional stand-by capacity will only be required at times of high electricity demand and low intermittent renewable supply. At other times (for example, during low electricity demand periods), additional stand-by capacity will not be required, as there will be sufficient conventional plant available to meet demand irrespective of the contribution of intermittent renewables.

Calculating Additional Stand-by Capacity

  22.  Consider Figure 4, which shows the peak hourly electricity demand experienced during each day of one year. The highest hourly peak demand experienced in any single day was 51,364MW (occurring on 3 January at 6pm). Under a system with no intermittent renewable generation, a minimum of 51,364MW of generating capacity would therefore have been needed to meet the maximum demand (shown by the dashed line on Figure 4), not including additional capacity currently maintained as a safety margin for plant failure.

  23.  Now consider the same demand pattern under a system with 10 per cent of annual electricity demand being met by intermittent renewables. For this system, around 3,500MW of conventional generating capacity would be decommissioned (assuming a 100 per cent capacity factor for the conventional plant), leaving 47,864MW of conventional capacity (shown by the black line on Figure 4). The decommissioned conventional plant would be replaced with intermittent renewables capable of delivering the same amount of annual electricity (for example, around 10,000MW of wind turbines at 35 per cent capacity factor).

  Figure 4—Peak Daily Demand in England and Wales

  24.  At this point, conventional plant would be able to meet demand up to 47,864MW, with any demand over this level being met by intermittent renewable generation. Clearly the question arises—what happens if the intermittent generation fails? Figure 5 shows the net peak daily demand, ie the peak daily demand less the contribution from intermittent renewables (for simplicity, the net peak demand shown is the highest value occurring on that day in any year 1980-2000).

  25.  From Figure 5 it is clear that the combination of wind, solar PV and dCHP intermittent renewables are highly reliable in meeting peak demand. Indeed, for the period 1980 to 2000, the absolute worst-case hour where intermittent renewables, combined with conventional generating capacity, did not meet demand would have resulted in a stand-by capacity requirement of just 400MW of additional convention generating capacity. Not providing this additional stand-by capacity would have resulted in demand not being fully met for two hours over the 21-year period.

  Figure 5—Peak Daily Demand in England and Wales

  26.  By comparison, if wind power systems were the sole provider of intermittent electricity, meeting 10 per cent of annual England and Wales electricity demand, then the additional stand-by generating capacity would be 3,135MW. Thus, by combining different renewable technologies across a range of locations, an 87 per cent reduction in stand-by capacity has been achieved.

ADDITIONAL BENEFITS

  27.  There are system benefits in additional to reduced stand-by requirements that are gained through this approach to renewables development, including:

—  Reduced incidence of over-generation of electricity (due to better matching between intermittent supply and demand patterns);

—  Reduced load-following requirements for conventional generating systems (due to lower overall system intermittency), and

—  Reduced peak grid demand levels experienced by the network (due to solar panels on domestic roofs producing electricity at point of demand).

CONCLUSION

  28.  The implementation of a demanding renewables programme by the Government is welcomed, however the Government needs to be aware that a poorly planned, ad hoc approach to achieving its renewables targets will result in consumers incurring additional costs, and the Government underachieving on its CO2 emissions reductions from the electricity generating sector.

  29.  This submission has demonstrated that substantial reductions in stand-by capacity requirements can be achieved through the implementation of renewable energy systems in a coherent, planned strategy, with the need for conventional stand-by capacity reduced to a minimum when a range of intermittent technologies are installed in a range of geographic locations.

October 2003

2   This equates to around 2,500 2.5MW offshore wind turbines. Back