Influence of the CO 2 emission factor value on the result of the load distribution analysis between the hybrid PV/WT/FC system and the electric power system

. This article presents the results of the multicriteria analysis of loads distribution between hybrid power generation system consisting of PV/WT/FC and the electricity system. The simulations were performed for power systems with different structures of the manufacturing sector, and so with different values of the factor of CO 2 emission. A decision-making criteria, which were used to compare the relevant scenarios, have been defined.


Introduction
One of the indicators characterizing the electricity system of the country is the unit rate of carbon dioxide emissions that tells how much CO2 accompanies the production of 1 MWh of electricity. This indicator characterizes the structure of the manufacturing sector of the electricity system. In the Polish power supply system, which is based on the carbon sources, carbon dioxide emission ranges from 700 to 800 kgCO2/MWh, depending on the usage degree of wind power for the energy production process. For comparison, the annual average rate in Germany is 560, however, its instantaneous value fluctuates within the limits from 300 kgCO2/MWh to 700 kgCO2/MWh depending on the use of renewable sources installed in German system in accordance with the Energiewende policy [1]. Czech electricity system, where two nuclear power plants are installed is characterized by the value at 450 kg/MWh. Systems based on renewable sources, for example, Swedish and those nuclear -French, are characterized by the indicator at the level of 40 kgCO2/MWh and 70 kg/MWh. An indisputable is the fact that the emissivity factor values should be minimized. Therefore, countries with systems based on conventional sources, must develop strategies to transform the manufacturing sector.
In publications concerning CO2 emissions related with electricity generation authors calculate the values of emission indicators for the system of specific manufacturing structure [2,3,4]. In case of distributed generation, the CO2 emission indicator for hybrid power generation system is formulated [5,6,7]. The purpose of this paper is to present the results of studies on the effects of the emission of CO2 on multi-option analysis of load distribution between the electricity system and PV/WT/FC hybrid power generation system -HPGS. The aim of presented research is to define the optimal exploitation strategy for hybrid power generation system in the power system of certain manufacturing structure.

A DESCRIPTION OF THE PROBLEM
Flowchart of analyzed system is shown in Figure 1. The balance of power in the system, depends on the ratio of power generated by renewable energy sources to the demand of the recipient. In the case where the demand is less than the power generated balance sheet equation takes the form [6]. sys comp elek WT PV load P P P P + P = P    (1) where: Pload-power consumed by the recipient, PPV -power generated by the photovoltaic installation, PWT -power generated by the wind plants, Pelek -power consumed by electrolyzer, Pcomp -power consumed to drive the compressor, Psys -power drawn from the power system.
If the demand exceeds the power produced by sources the receiver power describes the formula: where: PFC -power generated by the fuel cell stack. The analysis assumes the profile of municipal customer with a maximum power consumption of 80 kW and an annual electricity demand in the amount of 478 MWh. Simulated scenarios of covering the load are shown in Figure 2. In the S-I scenario, the basic load is covered by the electricity system while the remaining part of the demand is covered by hybrid power generation system. In the S-II scenario, the ordered power from the power system is 20 kW, in the S-III scenario -30 kW and in the S-IV scenario -40 kW. As the decision criteria for the multi-criteria analysis the following indicators has been adopted: k1 -unit usage of additional hydrogen by the hybrid system, k2 -unit cost of energy production by the hybrid system, k3 -the indicator of carbon dioxide emission by hybrid power system and electric power system, k4 -the capacity of utilization of power ordered from the electric power system. where: Ke -operating costs of hybrid system, EHPGS -the amount of energy produced by hybrid power generation system ] kWh where: Esys -energy input from the electricity system, WECO2 -indicator of carbodioxide emission in electricity system, ρH2 -hydrogen density, MCO2/MH2 -the amount of carbon dioxide formed in the production of 1 kg of hydrogen in the steam reforming process of hydrocarbons ] kWh kWh where: Pord -maximum input power from the power system by the consumer at time t = 8760 h.
Multi-criteria analysis is a mathematical method which allows to select the most favorable solution, the so-called. scenario, in the light of adopted criteria. One of the methods of multi-criteria analysis is "a compromise programming method" [8]. It allows to rank examined scenarios in order from the shortest distance from the socalled X' destination. The destination coordinates are equal to the maximum value of the adopted standardization scale, i.e. always takes the best value. The method of "compromising programming" allows to assign weights to each decision-making criteria, i.e. the decisionmaker has the ability to take into account in the calculations validity of some parameters, especially important for him.
In order to bring the values of all the criteria for a unified scale, the normalization criteria has been used in accordance with the equation (7).  -table 1, values of decision-making criteria -table 2 and normalized values of decision-making criteria -table 3. Table 1. Equipment forming a hybrid power system for scenarios S-I…S-IV.

Devices in hybrid system
Scenarios of power system loads.   The simulations were conducted for the equal weight value of decision-making criteria and for double overweighting of the given decision -making criteria relative in terms of the others. The calculations were performed for the following values of CO2 emission indicators: accordingly 800 kgCO2/MWh, 600 kgCO2/MWh, 400 kgCO2/MWh, 200 kgCO2/MWh, 70 kgCO2/MWh. The research assumes that the value of the pointer of the CO2 emissions associated with the hydrogen production in steam reforming process to HPGS is at the level of 13.7 kg.

Analysis results
The table shows the results of the multi-variant analysis of loads distribution between hybrid power generation system and electricity system which are shown in the tables 4-8.   In case of balanced weights distribution of decision-making criteria, scenario S -IV is indicated for value α equal to 1 and 2, as accepted for CO2 at 400 kgCO2/MWh.
Doubling the overweighting of energy criterion k1, which is the unit fuel consumption (hydrogen) by hybrid power generation system, the 4th scenario is chosen for all the considered values of the indicator of CO2 emissions equal to 800 kg/MWh. It is due to the fact that the k1 criterion prefers S-IV scenario, in which the hybrid power generation system will generate the least energy.
At overweighting of k2 cost criteria, the unit cost of electricity generation, S-IV scenario, is chosen for both values of an aggregate criterion α = 1 and α = 2, for values of the CO2 indicator at 70 kgCO2/MWh. This is related to the fact that for such a low value of the indicator of CO2 emissions in the electricity system, HPGS work with the use of additional hydrogen emits more CO2. Therefore, despite the largest unit costs of work of HPGS, the preferred scenario is S-IV.
The environmental criterion k3, prefers the S-I scenario, in which during the process of electricity production total carbon dioxide emissions should be as low as possible. The research has shown that the scenario S-IV is chosen for both values of an aggregate criterion α = 1 and α = 2, for values of the CO2 indicator at 200 kgCO2/MWh, however, it is worth mentioning that for the factor at 400 kgCO2/MWh the S-I scenario is not pointed as the best variant of loads distribution.
The system criterion of k4 prefers the S-I scenario, in which the electricity system is working at the base load. A reduction in the value of emission factor in the system to 600 kgCO2/MWh causes that for α = 1 is pointed S-II scenario. Further reducing of the value of the emission factor causes that the best variant, despite the double overweighting of k4 criterion to the others, the S -IV scenario is chosen.

Conclusion
The performed research allow for the formulation of the following conclusions:  factor value of CO2 emission significantly affects the result of the multi-variant analysis of loads distribution between hybrid power generation system and electricity system. Reducing the value of factor emission in power generation system causes as acceptable scenarios are indicated those where the power system has a larger part in covering the load profile of the recipient,  in the Polish power generation system, characterized by the index of carbon dioxide emission at the level of approximately 800 kgCO2/MWh, the load distribution between the HPGS and power generation system needs to be carried out according to S-I or S-II scenario. In electricity systems at the value of a pointer at the level of the 600 kgCO2/MWh this distribution should be carried out according to the S-II scenario, however in systems with the value of indicator at 400 kgCO2/MWh and lower, the load distribution is recommended according to the S-IV scenario,  the proposed analysis model can be a starting point to optimize the work of hybrid power generation systems based on sources of stochastic nature of work in the electricity system at varying structure of the manufacturing sector.