This tutorial demonstrates different allocation methods for specific CO2 emissions of electricity and heat from CHP plants. The key figures CO2 Factor, Primary Energy Factor, Levelized Cost of Energy, and Self Sufficiency Rate are considered separately for electricity and heat generation.
Allocation Methods
For the calculation of the energetic and ecological Key Figures, the CO2 emissions or the primary energy input are assigned to a specific form of energy, e.g., electricity or heat production (see also the tutorial on the use of the key figures). In the case of CHP plants, there is the fundamental problem of allocating the emissions caused to both co-products — electrical work and useful heat. There is no standardized calculation rule for the exact allocation of emissions to an energy form, but different equally valid calculation methods. In the following, two methods for calculating CO2 emissions and the primary energy input of electricity and heat from cogeneration plants are presented.
Finnish Method
In the Finnish method, also called alternative generation method, the allocation of CO2 emissions from CHP plants to the two forms of energy produced is carried out using reference efficiencies, it is therefore also called the reference efficiency method (German: Referenzwirkungsgradmethode). The application of this method is demonstrated in variants 1 and 2 of the tutorial.
Variant 1: Common Fuel Purchase
Variant 1 includes several heat and CHP plants supplied by the same fuel supplier. On the scheme, two Key Figure components for the CO2 Factor and two for the Primary Energy Factor have been added. One of each is used for the calculation of the electricity-related key figure and the other for the calculation of the heat-related key figure.
Electricity
To calculate the electricity-specific key figures, first select the Electricity Demand as the reference variable in the input form. With a click on Import, you then load all elements. The weighting factors are used: the corresponding checkbox is ticked. The weighting factor for the Electricity Supplier is specified as 1, because all the CO2 emissions and primary energy generated there are assigned to electricity.
To calculate the weighting factor for the fuel, a more complex method must be used here, because the fuel is partly allocated to heat generation and partly to electricity generation. In this example, the weighting factor is calculated using the components of the Programmable Control. In the form of the key figure component, the pre-calculated value is therefore displayed in blue.
The weighting factor is calculated according to the following formulas:
The total fuel consumed by the cogeneration unit \( f_{CHP}\):
\( f_{CHP} = \frac {consumed\ fuel\ power\ (CHP)} {supplied\ fuel\ power\ (supplier)} \)The fuel share of the CHP unit for electricity generation \( f_{CHP,el}\):
\( f_{CHP,el} = (1-PES) \cdot \frac {\eta_{el}} {\eta_{el,ref}} \cdot f_{CHP} \)Primary energy saving \( PES \):
\( PES = 1- \frac {1} {\frac {\eta_{el}} {\eta_{el,ref}} + \frac {\eta_{therm}} {\eta_{therm,ref}} } \)Specific CO2 emissions of electricity:
\( = \frac {CO2\ emission\_fuel\ supplier\ \cdot f_{CHP,el}\ +\ CO2\ emission\_electricity\ supplier} {total\ power\ demand} \)The values of the reference efficiencies are generally determined, but may differ depending on the federal state or organization. In this example, the values of the Arbeitsgemeinschaft Energiebilanzen \( \eta_{el,ref} = 40 % \) and \( \eta_{therm,ref} = 80 % \) are used.
In the scheme “Fuel share electric” this calculation is implemented using the components of the programmable control. The annual thermal and electrical efficiency of the cogeneration unit is calculated by taking the integrals of the respective powers from the control components Power_Input-A (Power Algebraic) and dividing them by each other. The thermal and the electrical reference utilization factor are each specified as a fixed value using the Fixed_Value_Input-A (Fixed Value (Algebraic)) component. With the help of the component Power_Input-A (algebraic), the consumed fuel power of the CHP and the delivered fuel power of the fuel supplier are obtained. With the Integrator-A component, the integrals are calculated, and with the Maximum-A component, only the maximum, the final integral value, is used. The share that the CHP unit has in the fuel purchased from the gas supplier is calculated in the component Fuel share_CHP unit and is included as a preset value in the corresponding key figure components.
The following figure shows the control scheme “Fuel share electric”.
Heat
The heat-specific key figures are calculated in the same way as the electricity-specific key figures. Analogous to the calculation of the specific CO2 emissions of electricity, the specific CO2 emissions of heat are calculated. The fuel share of the boiler must also be taken into account, as this is also included in the heat generation.
The weighting factor is calculated according to the following formulas:
The consumed fuel power share of the CHP:
\( f_{CHP} = \frac {consumed\ fuel\ power\ (CHP)} {supplied\ fuel\ power\ (supplier)} \)The consumed fuel power share of the hot water boiler:
\( f_{boiler} = 1- f_ {CHP} \)The consumed fuel power share of the CHP for heat generation \( f_{CHP,therm}\):
\( f_{CHP,therm} = (1-PES) \cdot \frac {\eta_{therm}} {\eta_{therm,ref}} \cdot f_{CHP} \)Primary energy saving \( PES \):
\( PES = 1- \frac {1} {\frac {\eta_{el}} {\eta_{el,ref}} + \frac {\eta_{therm}} {\eta_{therm,ref}} } \)Specific CO2 emissions of heat:
\( = \frac {CO2\ emission\_fuel\ supplier\ \cdot\ (f_{CHP,therm} + f_{boiler})} {total\ heat\ demand} \)The following figure shows the control scheme “Fuel share thermal”.
Variant 2: Separate Fuel Purchase
Variant 2 considers an energy system in which the heat and CHP plants are supplied with fuel separately.
If the fuel purchase of the heat generator is separated from that of the CHP plant in the TOP-Energy scheme, the extra calculation of the total consumed fuel share of the CHP can be omitted.
Electricity
To obtain the electricity-specific indicators, the specific CO2 emissions or the primary energy input for electricity generation are calculated using the following formulas.
The fuel share for power generation by the CHP \( f_{CHP,el} \):
\( f_{CHP,el} = (1-PES) \cdot \frac {\eta_{el}} {\eta_{el,ref}} \)Specific CO2 emissions of electricity:
\( = \frac {CO2\ emission\_fuel\ supplier\_CHP\ \cdot\ f_ {CHP,el}\ +\ CO2\ emission\_electricity\ supplier}{total\ electricity\ demand} \)The calculation is implemented in the “Fuel share electric” control scheme as follows.
Heat
Analogous to the calculation of the specific CO2 emissions or the fuel consumption of the electricity, the specific key figures of the heat are computed. The calculation is based on the following formulas.
The fuel share for heat generation by the CHP \( f_{CHP,therm} \):
\( f_{CHP,therm} = (1-PES) \cdot \frac {\eta_{therm}} {\eta_{therm,ref}} \)The fuel share for heat generation by the boiler \( f_{boiler} \):
\( f_{boiler} = 1 \)Specific CO2 emissions of heat:
\( = \frac {CO2\ emission\_fuel\ supplier\_CHP\ \cdot\ f_ {CHP,el}\ +\ CO2\ emission\_electricity\ supplier}{total\ electricity\ demand} \)The calculations are implemented in the “Fuel share thermal” control scheme as follows:
Variante 3: Method of IEA
Variant 3 illustrates how the International Energy Agency (IEA) method is applied. The IEA method is very similar to the Finnish method. Unlike the Finnish method, the IEA method does not consider reference efficiencies.
Electricity
In the context of determining the electricity-specific key figures, the CO2 emissions of electricity are calculated according to the following formula:
The fuel share for power generation by the CHP \( f_{CHP,el} \):
\( f_{CHP,el} = \frac {\eta_{el}} {\eta_{el}\ +\ \eta_{therm}} \cdot f_{CHP} \)Specific CO2 emissions of electricity:
\( = \frac {CO2 emission\_fuel\ supplier\_CHP\ \cdot\ f_ {CHP,el}\ +\ CO2\ emission\_electricity\ supplier}{total\ electricity\ demand} \)
Heat
As an example for the determination of the heat-specific key figures, the formulas for the calculation of the CO2 emissions of heat are given below.
The fuel share for heat generation by the CHP \( f_{CHP,therm} \):
\( f_{CHP,therm} = \frac {\eta_{therm}} {\eta_{el}\ +\ \eta_{therm}} \cdot f_{CHP} \)The fuel share for heat generation by the boiler \( f_{boiler} \):
\( f_{boiler} = 1 \)Specific CO2 emissions of heat:
\( = \frac {CO2\ emission\_fuel\ supplier\_CHP\ \cdot\ f_ {CHP,therm}\ +\ CO2\ emission\_fuel\ supplier_boiler}{gesamter\ electricity\ demand} \)The following figure shows the control scheme “Fuel share thermal” of variant 3.
