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 Research by TU Delft proves that Dutch power stations are able to cope at any time in the future with variations in demand for electricity and supply of wind power, as long as use is made of up-to-date wind forecasts. PhD candidate Bart Ummels also demonstrates that there is no need for energy storage facilities. Ummels will receive his PhD on this topic on Thursday 26 February. Wind is variable and can only partially be predicted. The large-scale use of wind power in the electricity system is therefore tricky. PhD candidate Bart Ummels MSc. investigated the consequences of using a substantial amount of wind power within the Dutch electricity system. He used simulation models, such as those developed by Dutch transmission system operator TenneT, to pinpoint potential problems (and solutions).
His results indicate that wind power requires greater flexibility from existing power stations. Sometimes larger reserves are needed, but more frequently power stations will have to decrease production in order to make room for wind-generated power. It is therefore essential to continually recalculate the commitment of power stations using the latest wind forecasts. This reduces potential forecast errors and enables wind power to be integrated more efficiently.
Ummels looked at wind power up to 12 GW, 8 GW of which at sea, which is enough to meet about one third of the Netherlands demand for electricity. Dutch power stations are able to cope at any time in the future with variations in demand for electricity and supply of wind power, as long as use is made of up-to-date, improved wind forecasts. It is TenneTs task to integrate large-scale wind power into the electricity grid. Lex Hartman, TenneTs Director of Corporate Development: "in a joint effort, TU Delft and TenneT further developed the simulation model that can be used to study the integration of large-scale wind power. The results show that in the Netherlands we can integrate between 4 GW and 10 GW into the grid without needing any additional measures.
Surpluses
Ummels: Instead of the common question What do we do when the wind isnt blowing?, the more relevant question is Where do we put all the electricity if it is very windy at night?. This is because, for instance, a coal-fired power station cannot simply be turned off. One solution is provided by the international trade in electricity, because other countries often can use the surplus. Moreover, a broadening of the opening hours of the international electricity market benefits wind power. At the moment, utilities determine one day ahead how much electricity they intend to purchase or sell abroad. Wind power can be better used if the time difference between the trade and the wind forecast is smaller.
No energy storage
Ummels research also demonstrates that energy storage is not required. The results indicate that the international electricity market is a promising and cheaper solution for the use of wind power.
Making power stations more flexible is also better than storage. The use of heating boilers, for instance, means that combined heat and power plants operate more flexibly, which can consequently free up capacity for wind power at night.

Apart from the sun, solar power systems cannot work without a reflow protection diode between the solar panel and the energy store. When current flows into the store, there is a potential drop across the diode which must be written off as a loss in energy. In the case of a Schottky diode, this is not less than 0.28 V at nominal current levels, but will rise with higher ones. It is clear that it is advantageous to keep the energy loss as small as possible and this may be achieved with external circuitry as shown in the diagram. The circuit is essentially an electronic switch consisting of a high precision operational amplifier, IC1a, a Type OP295 from Analog Devices, and a MOSFET, T1.

This arrangement has the advantages over a Schottky diode that it has a lower threshold voltage and the lost energy is not dissipated as heat so that only a small heat sink is needed. When the potential at the non-inverting input of the op amp, which is configured as a comparator, rises above that at the inverting input, the output switches to the operating voltage. The transistor then comes on, whereupon light-emitting diode LD1 lights. Diode D3 clamps the inputs of IC1a so that the peak input voltage cannot be greater than half the threshold voltage, provided the values of R3 and R4 are equal.


The op amp provides very high small-signal amplification, a small offset voltage, and consequent fast switching. The MOSFET changes from on to off state and vice versa at drain -source voltages in the microvolt range. In the quiescent state, when UDS is 0 V, the transistor is on, so that LD1 lights. The operating voltage (C–A) may be between 5 V (the minimum supply for the op amp and the input control potential, UGS, of the transistor) and 36 V (twice the zener voltage of D1). Zener diode D1 protects the MOSFET against excessive voltages (greater than ±20 V). Diode D3 and resistors R3 and R4 halve the potential across the inputs of the op amp.

This ensures that operation with reversed or open terminals is harmless. The substrate diode of the MOSFET is of no consequence since it does not become forward biased as long as the forward voltage, USD, of the transistor is held very low. The on -resistance, RSD(on), of the transistor is only 8 mΩ and the transistor can handle currents of up to 75 A. When the nominal current is 10 A, the drop across the on-resistance is 80 mV, resulting in an energy loss of 0.8W. This is low enough for a SUB type with a TO263-SMD case to be used without heat sink. When the current is 50 A, however, it is advisable to use a SUP type with a TO220 case and a heat sink since the transistor is then dissipating 12.5 W.

Even then, the voltage drop, USD = 0.32 V is significantly lower than that across a Schottky diode in the same circumstances. Moreover, owing to the high precision of IC1a, a number of transistors may be used in parallel. The circuit proper draws a current of 150 µA when only one of the op amps in the OP295 is used. An even lower current is drawn by the alternative Type MAX478 from Maxim. However, the differences between these two types are only relevant in the low current and voltage ranges. Both have rail-to-rail outputs that set the control voltage accurately even at very low operating voltages.

This is important since the switch-on resistance of MOSFETs is not constant: t drops significantly with increasing gate potentials and decreasing temperature. A experimental circuit may use an LM358 op amp and a Type BUZ10 transistors, but these components do not give the excellent results just described.