Greenhouse gases that are most relevant to energy system analysis are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O); however, CO2 is considered the main contributor to global warming. For this reason, CO2 emission effects are only considered in the present analysis. The amount of fossil carbon emission avoided by using a renewable resource instead of a fossil fuel power generation technology depends on the fossil fuel type that is avoided and on the conversion technology that would have been used to make the power from that fossil fuel. Replacing a higher CO2 emitting plant with a lower or non-CO2 emitting plant results in decreased CO2 addition to the environment. This difference in the amount of CO2 emitted is referred to as CO2 avoided emission. The CO2 emission reduction or CO2 emission avoided is generally defined as the difference between emissions generated by conventional systems and emissions generated in the production of the solar system over the lifetime of the system (25 years). A theoretical model is developed to calculate the CO2 avoided emission achieved by using solar heating and cooling system.
The energy generation cost (Eg) of the solar heating and cooling system in $/kW h is calculated from the following equation:
g pg where Chc is the annual expenses of the solar heating and cooling system ($/year) and Pg is the annual power generation in kW h.
In the present analysis, avoided CO2 emission (EA) in (tonne CO2) is mainly the CO2 emissions which generated by conventional systems and can be expressed by the following expression:
where Pg is the power generation (kW h) and FE is the plant emission factor (tonne CO2/kW h).
The cost of CO2 gas mitigation using renewable energy technologies depends on both the difference between the generation costs of the renewable energy systems and the costs of conventional energy generation and the carbon emissions that are displaced by the renewable energy generation. The mitigation costs are usually expressed in units of the cost per unit fossil carbon emissions that are avoided.
The present model also calculates the annual reduction in CO2 emissions estimated to occur if the proposed solar heating and cooling system is implemented instead of alternative conventional energy systems assuming that each kW h generated by solar system substitutes 1 kW h produced by the conventional energy systems. To perform CO2 emission reduction analysis for the solar system, one needs to define the baseline (also called base case or reference case) electricity system. Often this will simply imply defining a conventional system and its associated fuel (oil, natural gas, coal). The default emission factors and conversion efficiencies of various fuel types (Energy Information Administration, 1999) are input to the model in each case. The model can calculate the CO2 emission reduction for each reference fuel type.
The cost associated with CO2 emission reduction is generally expressed in $/tonne of CO2 avoided. The CO2 avoided cost or reduction cost (CR) is calculated as follows (Narula, 2002):
where CC is the cost of kW h generated using conventional electricity system, Chc is the cost of kW h generated using solar heating and cooling system, EC is CO2 emissions in tonne/kW h from conventional system, Ehc is CO2 emissions in tonne/kW h from solar heating and cooling system. In another way around, CO2 emission reduction cost can be calculated by dividing the annual life cycle savings of the solar heating and cooling system by the net CO2 emission reduction per year, averaged over the system life.
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