This is the first of a series of periodic reports on the performance of the Gorona del Viento (GdV) hybrid wind-pumped hydro plant on the island of El Hierro in the Canaries, the ultimate goal of which is to demonstrate that a remote island that has historically been entirely dependent on diesel power can generate 100% of its electricity from renewables.
After a year-long test period the GdV plant went into full operation on June 27, 2015. According to grid data published by the Red Eléctrica de España (REE) renewable energy from GdV has supplied 42% of the electricity sent to the El Hierro grid in the 96 days of operation since then (from June 27 through September 30) with the remaining 58% coming from diesel:
- Diesel: 7,457 MWh (58%)
- Renewables: 5,428 MWh (42%)
- Total: 12,885 MWh
Figure 1 shows average daily diesel and renewables generation since June 27. The upper limit of the bars tracks El Hierro’s electricity demand usually within a couple of tenths of a megawatt:
Figure 2 plots the daily percentage of El Hierro’s grid demand supplied by renewables. The highest percentage (71.5%) was recorded on August 6 and the lowest on September 24 (0.4%). The contribution of renewables began to decline late in August and since September 11 renewables have supplied only 12% of grid demand. The decline coincides with decreasing wind speeds.
Figure 2: Percentage of renewable electricity supplied to the El Hierro grid, June 27 through September 30, 2015, daily REE averages
Plant design and operation
The GdV plant consists of:
1. An 11.5MW wind farm with five 2.3MW Enercon E-70 wind turbines.
2. The Llanos Blancos diesel plant, containing six units aggregating 11.36MW.
3. A pumped hydro system, consisting of a 380,000 cu m upper reservoir, a 150,000 cu m lower reservoir, a pumping station with 6MW capacity and a generation plant with a nominal capacity of 11.3MW, although the use of the turbines for phase compensation reduces the net capacity to 9.2MW. The round-trip efficiency of the pumped hydro system is given as 60%.
Since all three units can generate power simultaneously the installed generating capacity on El Hierro is 32.06MW, comfortably in excess of average demand (~5.4MW) and peak demand (~7.6MW).
Plant layout is shown in Figure 3:
Figure 3: Gorona del Viento plant layout
The plant was designed to operate as follows, although as discussed later it has not operated this way in practice. Power from the wind farm is sent directly to the grid. When wind generation exceeds demand the surplus power is used to pump water from the lower reservoir to the upper reservoir, and the water is later released back down through the hydro turbines to cover demand shortfalls during low wind periods. The diesel plant provides backup power when the wind farm and the hydro system are unable to fill demand. More information on plant design and operation is provided in the links below:
Plant operation to date
Details of actual plant operation have been reconstructed as far as is possible from the REE grid data. (Information on metering practices and reservoir balances has been requested from REE and GdV but so far none has been supplied).
REE provides values at ten minute intervals and expressed to the nearest 0.1MW for demand, diesel generation, wind generation and hydro generation sent to the El Hierro grid. The sum of diesel + wind + hydro tracks demand but the hydro generation value is usually negative, signifying that power was taken from the grid, not sent to it. Here is a typical REE entry:
- August 13, 0020: Diesel 1.0MW, wind 7.3MW, hydro -2.6MW, demand 5.6MW
These numbers can be reconciled only if REE’s 7.3MW of wind generation includes both wind power sent to the grid and wind power consumed elsewhere, with this power entered as a negative value because it represents load rather than generation. If this is what is happening then what the above numbers actually show is:
- Diesel to grid 1.0MW, wind to grid 4.7MW, total to grid 5.7MW, wind consumed elsewhere 2.6MW
There is no ambiguity when there is no negative hydro. Diesel +wind + hydro matches demand:
- August 13, 0750: Diesel 1.6MW, wind 0.0MW, hydro 3.5MW, demand 5.0MW
- August 13, 1430: Diesel 5.1MW, wind 0.9MW, hydro 0.9MW, demand 6.8MW
As to where the negative wind was consumed, the logical supposition is that it was consumed pumping irrigation water up to the upper reservoir, which was the original purpose of the GdV project and is still a “primary concern“:
Gorona del Viento was originally conceived mainly as a way to pump fresh water up to a high-altitude reservoir, from where it could be distributed across the island. Its potential as a power supply came almost as an afterthought. Even in the final plan, water distribution is still a primary concern. The uppermost reservoir, holding 380,000 m3 of water, is oversized so that it will deliver irrigation when the lower pool is full.
As illustrated in Figure 4, the power consumed in pumping water to the upper reservoir greatly exceeds the power generated by the hydro turbines, indicating that almost all of the water pumped to the upper reservoir was taken for irrigation or other uses.
Figure 4: Wind power consumed in pumping and hydro power sent to the El Hierro grid, June 27 through September 30, 2015, daily REE averages
Average daily generation from all sources since startup with irrigation pumping included is as shown in Figure 5. (Note that except for a 2-hour outage on August 9 diesel generation has never fallen below ~1.5MW. The reasons for this are unknown):
Figure 5: Allocation of power generated by Gorona del Viento, June 27 through September 30, 2015, daily REE averages
If the above interpretation is correct – and for the purposes of further discussion we will assume that it is – then GdV has been operating partly as a power plant and partly as a pumping plant. The power consumed in pumping, however, was not wasted. If it had not come from the wind farm it would have had to come from the grid, so effectively what the pumping has done is shave 1.6MW off island demand, which would have increased from ~5.4MW to ~7MW if the grid had supplied the power. Recalculating El Hierro’s electricity consumption with this added power included in both generation and demand increases the contribution of renewables from 42% to 54%:
- Diesel 7,457 MWh (46%)
- Renewables: 9,179 MWh (54%)
- Total: 16,635 MWh
The addition of pumping has, however, further complicated the “merit order” protocol under which wind power is allocated. The protocol must now balance wind generation, electricity demand, reservoir status, desalinated water availability and irrigation needs all at the same time. Exactly how this is done cannot be determined from the REE data, but it appears that irrigation pumping takes priority, with wind-to-grid second and hydro generation third. One impact has been to sideline the hydro plant, which has delivered small amounts of power to the grid at irregular intervals instead of functioning in its intended load-following mode. (A 2012 production study estimated that the hydro plant would send 5.6GWh/year to the grid, but in the 96 days since startup it has sent only 533MWh, equivalent to only 2 GWh/year.)
Low-wind conditions and energy storage
The 2012 production study linked to above estimated that the GdV wind farm would deliver 25GWh/year to the grid. This represents a capacity factor of 25%, a reasonable number for an onshore wind farm and one which may well be achieved in GdV’s first year of operation (the capacity factor to date is 32%). As noted in the introduction, however, wind generation has been decreasing since late August, and since September 12 the wind farm capacity factor has fallen to 7%. Figure 6, which compares GdV wind generation with wind speeds recorded at El Hierro airport 3km to the northeast confirms that this is a result of decreased wind speeds and not turbine outages. (Wind turbines typically generate no power at wind speeds of less than 3.5m/s.)
Figure 6: Gorona del Viento wind generation versus wind speeds at El Hierro airport, June 27 through September 30, 2015, daily REE averages
Figure 7 provides a more detailed picture of the ongoing low-wind period using 10-minute REE data. There have been a few days when the wind blew, although not strongly, but for most of the time wind generation has been low or even zero. For 22 hours between 0520 on September 29 and 0320 on September 30 GdV’s wind turbines generated no power at all:
Figure 7: Gorona del Viento wind generation and demand, June 27 through September 30, 2015, 10-minute REE data.
The gap between wind generation and demand shown in Figure 7 quantifies the deficit that pumped hydro generation would have had to fill before GdV could have supplied all of Hierro’s electricity between September 12 and September 30. It aggregates 1,200MWh. This far exceeds the ~270MWh storage capacity of GdV’s pumped hydro reservoirs, which are already as large as topography and foundation conditions will allow.
And the low-wind period is not over yet. If 2015 wind speeds replicate 2014 wind speeds it will continue until December. The wind may not in fact start blowing again in earnest until next summer (June, July and August are the windiest months on El Hierro).
Since startup the GdV plant has generated about as much renewable energy as it was expected to generate except for the shortfall in pumped hydro. The ongoing low-wind period, however, highlights the project’s Achilles Heel – energy storage. Previous Energy Matters posts have concluded that intermittent renewable energy will never be able to supply 100% of grid demand because of the prohibitively large amounts of energy storage needed to convert it into year-round dispatchable power, and GdV now provides real operating data to back this conclusion up.
This is not to say that GdV could never supply a substantial percentage of El Hierro’s electricity demand. But higher levels of renewables penetration can be achieved only by expanding the wind farm, using the diesel generators for load-following and wasting the surplus wind power the hydro system is too small to handle. Other previous Energy Matters posts have identified this approach as the only practicable one.