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.
Long-term perspectives for balancing fluctuating renewable energy sources 80 x Suitability: Trigeneration systems are an efficient way of recovering “waste” heat
(through absorption or adsorption chillers) or electricity (compression chillers) generated with CHP systems to produce cold as well as heat and electricity. The wind power peaks can be reduced when both types of chillers (heat-fired and electricity-fired) are installed in combination with the CHP plant. When an excess of electricity exists, compression chiller is used to produce cold, and when more electricity is needed, the surplus heat is used to produce cooling with absorption or adsorption chillers.
x The main characteristics of trigeneration are the high efficiencies and not very costly equipment and installation when a CHP plant exists
5.3.2 Comparison with CHP
Trigeneration systems are more efficient than CHP systems, as already stated in section 1.2. On the other hand, installation costs are greater, as cooling equipment is also needed.
CHP systems usually have a high thermal to electric ratio, so either they do not produce all the electricity required, or have excess heat. Since the trigeneration system uses more heat than CHP, more electricity demand can be satisfied locally and, thus, wind balancing ability is also greater. Besides, the increase on thermal demand will also probably increase the need for thermal storage, which also increases balancing ability.
5.3.3 Economic assessment: Costs, historical and future cost development
For the economic assessment, the same example as in Section 2 will be used. The analysis will consist is comparing the energy bill before the implementation of the trigeneration system and the energy bill after implementing the system. Then energy bill savings will be compared to the required investment, to determine whether savings pay for the investment.
Gas prices are updated every three months and electricity prices have been updated in the middle of 2006, but, for simplicity reasons, constant gas and electricity prices will be considered for the whole year. Prices to be taken into account are for the first quarter of 2006.
Before implementing the trigeneration system, gas consumption was 515 977 kWh/year, with a maximum demand of 450 kW. Electricity demand was 1 073 198 kWh/year, with a maximum of 230.20 kW. According to 0, gas price is 135.07 €/month and 0.021852 €/kWh (annual consumption between 500 000 kWh and 5 000 000 kWh). Therefore, gas bill before implementing the trigeneration system is:
Gas billbefore = 135.07 €/month * 12 months/year + 0.021852 €/kWh * 515 977 kWh/year Gas billbefore = 12 895.97 €/year
Regarding electricity bill, it must be taken into account the payment for accessing the grid and the payment for the electricity to be bought in the market. According to 0, energy and capacity terms for installations with a capacity above 15 kW depend on the time of consumption, as Table 5-17 shows:
Table 5-17. Acess tariff components
Period 1 Period 2 Period 3 Hours: Winter
Summer
18-22 9-13
8-18/22-24 8-9/13-24
0-8 0-8 Capacity term (€/kW/year) 21.551694 12.753270 2.767759 Energy term (€/kWh) 0.018980 0.017331 0.013712
Long-term perspectives for balancing fluctuating renewable energy sources 81 The market prices to be taken into account correspond to day-ahead market, as most transactions are made in that session. Data are obtained from the market operator’s homepage 0, and monthly average values for each hour are used. Values include the period between December 2005 and November 2006. Electricity bill is then obtained by adding the capacity term and the energy term, which includes the access tariff and the market price. For the capacity term, maximum demands in each period must be taken into account, which, in this case, are 231.04, 224.06 and 183.8, for periods 1,2 and 3, respectively. As a result, electricity bill was:
Electricity billbefore = 6 (Max.demand * Capacity term)period + 6 (Demand * Energy term)hour + 6 (Demand * Market price)hour
Electricity billbefore = 209 040.26 €/year
Consequently, total energy bill before implementing the trigeneration plant is the addition of gas bill and electricity bill:
Energy billbefore = Gas billbefore + Electricity billbefore Energy billbefore = 221 936.23€/year
Once the trigeneration plant is installed, gas consumption increases, but the tariff to be used is the same, so only consumption will be different from the previous bill:
Gas billafter = 135.07 €/month * 12 months/year + 0.021852 €/kWh * 935 422 kWh/year Gas billafter = 22 061.68 €/year
Now, electricity consumption is lower than in the case before, and also maximum hourly demands: 201.09, 205.71 and 179,94 for periods 1, 2 and 3. In this case, there is some extra electricity to be sold in the market at certain hours, so, at these hours, the value of the energy term of the access tariff will be zero. Electricity bill after inplementing the trigeneration system is therefore:
Electricity billafter = 6 (Max.demand * Capacity term)period + 6 (Demand * Energy term)hour, demand
+ 6 (Demand * Market price)hour, demand - 6 (Sales * Market price)hour, sales
Electricity billafter = 147 126.38 €/year
Adding up the two concepts, total energy bill after implementing the scenario will be:
Energy billafter = Gas billafter + Electricity billafter Energy billafter = 169 188.06€/year
Annual energy bill savings are obtained by substracting the new bill to the old bill:
Energy savings = Energy billbefore - Energy billafter = 221 936.23 €/year - 169 188.06 €/year Energy savings = 52 748.17 €/year
The equipment required for this installation includes the two microturbines and the absorption chiller. Installation costs for a microturbine and for an absorption cooling device are about 800
€/kWe for each 0. Therefore, total investment is:
Investment = InvestmentMicroturbine + InvestmentAbsorption = (800 €/kWe/unit * 135 kWe * 2 units) + (800 €/kWe/unit * 135 kWe * 2 units) = 216 000 €
Different investment criteria will be analysed:
1. Single payback period (SPP): 4.09years
748.17 52
000.00 216 Savings
Investment SPP
2. Net Present Value (NPV): Assuming a discount rate of 7%, accumulated NPV is positive in year 4, as Table 5-18 shows:
Long-term perspectives for balancing fluctuating renewable energy sources 82 Table 5-18. NPV analysis for the investment
Year 0 1 2 3 4
Investment costs -216 000.00 0.00 0.00 0.00 0.00
Savings 52 748.17 54 858.10 57 052.42 59 334.52 61 707.90
SUM -163 251.83 54 858.10 57 052.42 59 334.52 61 707.90
NPV -163 251.83 51 269.25 49 831.79 48 434.64 47 076.66
Accumulated NPV -163 251.83 -111 982.58 -62 150.79 -13 716.15 33 360.51 3. Internal Rate of Return (IRR): For the same conditions as for the NPV, the IRR value in
year 4 is 15.56%.
5.3.4 Environmental aspects
As described in “Yearly energy analysis” section (0), the use of trigeneration reduces the total energy requirements in the system, although local energy requirements increase. Therefore, the use of trigeneration reduces the emission of CO2 and other pollutants.
Another improvement of the environment refers to the reduction of peak power plants. As described in the “Economic assessment” section (5.3.3), the use of trigeneration reduces peak demands in the three billing periods. As a result, highly-pollutant peak-power plants can be switched off, so that the improvement is even higher.
The only adverse impact of trigeneration is that it increases local emissions. Nevertheless, the improvement in the system as a whole pays for it, unless the trigeneration plant is located in a highly polluted environment.
Long-term perspectives for balancing fluctuating renewable energy sources 83