The other subsystems of the Earth section of the model, population, capital investment and the fraction of capital devoted to agriculture, are the same as those in the original WORLD-2 model. The quantity of capital is calculated using a capital unit which is equivalent to about $5400 in the same way as in the original WORLD-2 model. The SPS section of the model is represented by the levels of SPS capital investment (SCI) and SPS total energy (STE). The value of STE increases in proportion to the value of SCI. The relation between STE and SCI are expressed by the transfer coefficient from capital to energy (TCCE), which represents the energy cost of SPS and will be described later. The SCI is determined by the difference between the SPS capital investment generation (SCIG) and the SPS capital investment discard (SCID). The SPS and Earth sections are connected through the following elements. (1) The capital investment in SPS from the Earth (CITS): CITS is related to the energy investment in SPS (Et) by equation (4) which is shown below. (2) The energy return from SPS to the Earth (ETE): the energy generated by SPS is supplied to the Earth whenever the energy generation ability of SPS reaches a unit value, which is 5GW. The energy generated by energy resources on Earth is replaced by this energy supplied from SPS (ETE) and the usage rates of fossil fuels on Earth (NRURO, NRURC, NRURG) decrease with the increase in ETE. The unit of ETE is converted from GW to natural resource units in the model. These relations are represented by the following parameters. (1) IYEAR (year): the start year of energy investment in SPS from the Earth. (2) RYEAR (year): the year when the SPS cost reaches the improved future value. The energy costs (Sm, Es) shown in Table 1 are assumed to increase linearly as follows, and RYEAR is given as a parameter which represents the condition of the cost increase. where i is the number of years after IYEAR. The values of energy costs are assumed to remain constant at the higher future values after RYEAR. These energy costs are converted to the transfer coefficient from capital to energy (TCCE, natural resource unit / capital unit) in the calculation by the following relation.
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