Conclusion

Long-term perspectives for balancing fluctuating renewable energy sources 76 Natural Gas

0 20000 40000 60000 80000 100000 120000 140000 160000

Jan Feb

March Apr May Jun July

Aug Sept Oct Nov

Dec Month

P (kW)

Demand

Figure 5-27: Monthly natural gas demand (Trigeneration system)

It is obvious that the natural gas consumption in summer is very small because in this season the thermal generation of the microturbine is very low.

Yearly energy analysis

Finally, if the values for all months provided in Table 5-11 are added, the energy balance for the whole year is obtained. These results, as well as the total natural gas demand for the whole year are presented below:

Table 5-13: Yearly energy balance (Trigeneration system)

Energy demand (KWh/year) Energy production (KWh/year) Electricity Thermal Gas Electricity Thermal

1 059 794 467 721 935 442 280 632 467 721

In order to determine the total amount of electricity to be bought on the market for the whole year, the electricity generation has to be subtracted from the total electricity production. That is:

1 059 794 –– 280 632 = 779 162 kWh/year.

Long-term perspectives for balancing fluctuating renewable energy sources 77 might even be a surplus that could be sold to the market. The amount of electricity bought is calculated subtracting the electricity generation of the Trigeneration plant to the total electricity demand of the building. The obtained results are graphically represented in the following figures. Only the three days selected in section 0 have been considered because they represent the behaviour of the system in the three time periods defined. In these graphs, the cyan line defines the amount of electricity imported from or exported to the market in each moment.

Negative values mean that there is a surplus of electricity generation.

March

-100 0 100 200 300 400 500

0:00 6:00 12:00 18:00 0:00

Time

P(kWh)

Electricity generation Electricity demand Thermal generation-demand Difference

Figure 5-28: Amount of electricity imported from/exported to the market on a winter day May

0 100 200 300 400 500

0:00 6:00 12:00 18:00 0:00

Time

P(kWh)

Electricity generation Electricity demand Thermal generation-demand Difference

Figure 5-29: Amount of electricity imported from/exported to the market on a spring/autumn day

Long-term perspectives for balancing fluctuating renewable energy sources 78

August

0 100 200 300 400 500

0:00 6:00 12:00 18:00 0:00

Time

P(kWh)

Electricity generation Electricity demand Thermal generation-demand Difference

Figure 5-30: Amount of electricity imported form/exported to the market on a summer day

Looking at the previous graphs it can be concluded that most of the time, the amount of electricity generated is not enough to cover the demand and, consequently, it is necessary to buy the difference in the market. The only exception occurs during winter months (January, February, March, November and December) where the energy production exceeds the demand during approximately two hours in the morning.

The following table shows a review of the daily electricity demands in both scenarios for the three days considered above, as well as the percentage of electricity that is saved to be bought in the energy market when the trigeneration system is employed:

Table 5-14: Daily savings on electricity

Electricity demand (kWh/day)

Month Period Conventional Trigeneration Saving (%)

March Winter 2 706.84 1 576.58 41.8

May Spring/Autumn 2 964.99 2 351.76 20.7

August Summer 3 220.14 2 909.84 9.6

It can be seen how the most important savings occur in winter months, where it is possible to reduce the amount of energy bought in the market in a 41.8 %. The reason is that the generation of the Trigeneration system is very big in this season, even exceeding the demand sometimes.

The following table shows the total yearly demands of electricity and natural gas for the two situations considered. It has to be noted that the electricity demand for the Trigeneration system is referred to the amount of energy that is bought in the market, that is, the difference between the electricity demand and the generation.

Table 5-15: Comparison between both scenarios

Demand (kWh/year) Gas Electricity Conventional installation 515 977 1 073 198 Trigeneration system 935 442 779 162

Long-term perspectives for balancing fluctuating renewable energy sources 79 Looking at this table it can be seen that the Trigeneration system reduces considerably the amount of electricity that is bought in the market. However, this reduction on the electricity demand implies an increase on the gas consumption of the building.

From the point of view of the system as a whole, it can be concluded that this kind of generation contributes to reduce the total consumption of primary energy. If it was considered that the energy production of the system as a whole is obtained using a thermal power plant of natural gas whose efficiency is 30% and that transmission and distribution losses of the system are equal to the 14% 0, the following values would be obtained:

Table 5-16: Comparison of the natural gas consumption of the system as a whole

Gas consumption (kWh/year) Electricity

demand (kWh/year)

Electricity production

(kWh/year) Whole

system Building Total Conventional 1 073 198 1 223 446 4 078 154 515 977 4 594 131

Trigeneration 779 162 888 244 2 960 814 935 442 3 896 255

That means, first the amount of natural gas has been calculated that the whole system has to burn in order to supply the required electricity demand. Then, the natural gas consumption of the microturbine installed in the building has been added to the previous value, obtaining as a result the total primary energy consumption. Looking at the results it can be concluded that the requirements of natural gas are smaller for the Trigeneration system. To be more precise, primary energy needs are reduced a 15%.

It is important to take into account that this reduction will contribute not only to provide economical savings, but also to reduce greenhouse gas emissions. This kind of plants has also the advantage of allowing a more reliable and safer energy supply.