Increasing of the share of renewable energy in the electricity production is an important issue everywhere in the world. The use of wind energy is one of the most attractive options here, especially in the countries with long coastline and islands. Estonia belongs to these countries and its wind potential is remarkable.
The introduction of wind energy has been in the focus of numerous conferences, seminars and energy plans and there has been several “waves” of interest of foreign investors during more than 10 years. Still there is only 2 MW of wind capacity installed today in Estonia.
The wind energy projects were simply infeasible during a long period due to low electricity price in existing power system and Estonia’s very limited possibilities to subsidize wind-generated electricity. In addition to that, multiple technical constraints existed as well: construction of windmills did not suite weak grids in the windy regions, absence of regulating power, needs for extensive electric network building, etc. Most of technical constraints are present also today. During last 10 years the electricity tariffs have raised several times and they continue to increase, the purchasing obligation and feed-in tariffs for electricity from renewables have written into legislation and construction of wind generators have improved substantially. This has caused recently new peak of interest in wind energy investments and even in local manufacturing of windmills to reduce the price of equipment. This interest is supported also by national target to increase the share of renewable energy in the electricity production up to 5,1% from gross inland consumption in 2010. This target was set to Estonia by European Union during the accession negotiations. The share was only 0,1% in 2001. Estonia has three possibilities to reach this heavy and expensive target: to develop biomass and wind power plants and to restore once existed small hydro plants. As the total hydro potential is only ca 30 MW, the biomass and wind have to fulfill major part of the goal.
The new “wind boom” have resulted in grid connection applications for more than 400 MW of wind capacity (ca 20% of total net capacity of existing power plants). It immediately raised questions: how many windmills can the present system integrate, what will be the costs of this expansion and what benefits can be achieved?
An interest in the development of a correct calculation method of environmental gain appeared at the Department of Electrical Power Engineering of Tallinn Technical University (TTU) already in the early 1990’s, when the feasibility studies of the wind energy project proposals in Estonia used an elementary calculation of environmental effect: installed capacity of windmills was multiplied to the load factor (usually overestimated) and to the number of hours in the year, then this energy production was subtracted from the annual production of the most polluting fossil power plant unit and finally, the corresponding reduction of air pollution was found. It was obvious that the calculation cannot be so simple .
The accounting of emission reductions is linear also in the MARKAL model  that has been used by TTU in the long term planning of Estonia’s energy sector and climate change studies since 1994 [3, 4]. MARKAL optimizes fuel mix and set of technologies, but it cannot consider additional emissions due to compensation of wind power fluctuations by fossil fuel power plants.
The basic concept of the methodology for calculation of the real environmental gain from wind power use was first published in Estonian language in  and developed further under the Estonian Science Foundation grant project . Now it is time to present this methodology for wider discussion.
Problem of balancing of wind power fluctuations It is well known that the wind power plants are almost uncontrollable, their power varies rapidly and frequently in a wide range, as their output power is a function of the wind speed in the third power, their production is hard to forecast and they cause various technical problems and additional investments in the system...........
Participation of thermal power plants in keeping the reserve capacity for wind turbines and in compensation of the fluctuations of wind power increases the fuel consumption and emissions substantially. Linear methods of calculation of emission reductions from wind energy use cannot consider this increase and herefore special methods for correct accounting of environmental gain have to be elaborated.
A simplified two-step method for calculation of real fuel consumption and emissions under absence of dynamic input-output characteristics of thermal power plants is proposed in this paper. Estonian case study shows that the integration of considerable capacity of wind turbines would increase the fuel consumption and emissions of thermal stations about 8-10%, which will reduce the environmental effect of windmills substantially. There can be situations where probably no environmental gain can be achieved at all.
It is vitally important to continue the discussion about the ability of power systems to integrate large amounts of wind power and to develop further the methods for the calculation of emission reductions.