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Energy Systems Language models of the resource base for the U.S. economy and of economic exchange were used, respectively, (1) to show how energy consumption and emergy use contribute to real and nominal gross domestic product (GDP) and (2) to propose a model of coupled flows that explains high correlations of these inputs with measures of market-based economic activity. We examined a third power law model of growth supported by excess resources and found evidence that it has governed U.S. economic growth since 1900, i.e., nominal GDP was best explained by a power function of total emergy use with exponent 2.8. We used a weight of evidence approach to identify relationships among emergy, energy, and money flows in the U.S. from 1900 to 2011. All measures of quality adjusted energy consumption had a relationship with nominal GDP that was best described by a hyperbolic function plus a constant and the relationship between all measures of energy consumption and real GDP was best described by a second order polynomial. The fact that energy consumption per unit of real GDP declined after 1996 as real GDP continued to increase indicates that energy conservation or a shift toward less energy intensive industries has resulted in lower fossil fuel use and reduced CO 2 emissions while maintaining growth in real GDP. Since all energy consumption measures versus real GDP deviated from a power law relationship after 1996; whereas, total emergy use did not, we concluded that total emergy use captured more of the factors responsible for the increase in real GDP than did energy measures alone, and as a result, total emergy use may be the best measure to quantify the biophysical basis for social and economic activity in the information age. The emergy to money ratio measured as solar emjoules per nominal $ followed a decreasing trend from a high of 1.01E+14 semj/$ in 1902 to 1.56E+12 semj/$ in 2011 with fluctuations in its value corresponding to major periods of inflation and deflation over this time.Keywords: emergy to money ratios, emergy of the energy consumed, emergy evaluation of the U.S., quality adjusted energy consumption, nominal and real GDP, money supplies www.frontiersin.org

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IntroductionAlthough self-pumping, two-phase solar heating systems have numerous advantages over mechanically pumped, sensible heat systems, experimental studies indicate that the design could be modified to improve thermal performance, reliability, and economics Neeper, 1985, 1986;. Efforts at optimizing the design and efficiency of these systems have been largely heuristic. Neither the approximate algebraic model of a single accumulator system presented by Neeper (1988) nor a more detailed simulation developed by Gari and Fathalah (1986) applies to the dual accumulator design.System operation is cyclic, progressing through a run or heat collection phase, a pressurizing phase, and a pump phase. During the run phase, shown in Fig. 1, liquid refrigerant (R-113) is gravity fed to the collector from the upper accumulator. Vapor travels downward to the condenser and liquid refrigerant collects in the lower accumulator tank. After a preset quantity of refrigerant (determined by the setting of the lower float switch in the upper accumulator) condenses, solenoid valve No . l closes, solenoid valve No. 2 opens, and the pressurizing phase shown in Fig. 2 begins. Since vapor is prevented from entering the condenser, pressures and temperatures in the collector and lower accumulator increase. When the pressure in the lower accumulator exceeds the pressure in the upper accumulator by an amount equivalent to the hydrostatic pressure of the liquid column in the return pipe, the pump phase