Several of the more commonly used energetics language symbols are illus- 

 trated in Figure 1. The water-tank-shaped symbol (A) represents an energy 

 storage. The lines intersecting the storage symbolize energy flow pathways 

 with flow in the direction of the arrows. The circle (B) is the symbol for an 

 energy source which suppl ies power to the model from outside the systems 

 boundary. The heat sink (C) is used to illustrate how waste heat or degraded 

 energy is removed from the system. 



The next three symbols (D, E, F) are group or subsystem symbols. These 

 symbols are used primarily to aid in model organization. The hexagonal symbol 

 (D) represents a self-maintaining consumer subsystem. A cow or city is an 

 example of a consumer system. Consumers require concentrated energy from pro- 

 ducers to operate, and feedback some energy to control the producer system. 

 The bullet-shaped symbol (E) represents a producer subsystem. Producers are 

 capable of upgrading dilute forms of natural energy such as sun, wind, and 

 rain into more concentrated forms of energy such as plant biomass. The use of 

 carbon from the atmosphere and nutrients from the soil by plants in the photo- 

 synthetic process is an example of a producer system. Producer and consumer 

 systems are coupled to process energy and cycle matter within energetics 

 models of systems of man and nature. The third group symbol (F) represents a 

 logic action. The logic symbol is used to diagram a process in which the out- 

 come has an off-on effect such as an electron. 



The transformation process is represented by G. Relative dilute energy 

 interacts with concentrated energy in the process symbol to produce some 

 intermediate product. This symbol is commonly called a production function. 

 An example would be the interaction of a plant with natural energy to produce 

 plant sugar or the interaction of materials, fuels, capital, and labor in a 

 city to produce a product. The energy and money transaction is represented by 

 H. The solid line represents the energy flow and the dashed line represents 

 the flow of money. The small circle is used to label the price (ratio of 

 money to energy). This symbol is often used at the system boundary to control 

 imports based on money stored in the system and collect money from exported 

 products. The last symbol (I) is a flow sensor which is used to monitor flows 

 of energy. 



PRINCIPLES OF ENERGETICS MODELING 



All energetics models, when designed properly, are consistent with the 

 first and second laws of thermodynamics. The first law of thermodynamics 

 states that energy is neither created nor destroyed; all systems of man and 

 nature conserve energy. This principle of conservation of energy is incor- 

 porated into energetics models by requiring that the sum of all flows into a 

 system, minus the energy flowing out, equal the net changes in energy storages 

 within the system or any part of the system. In developing an energetics 

 model that is consistent with the first law requirements, it is important that 

 all energy flows be measured in their heat equivalent value. 



The second law of thermodynamics pertains to the degradation of energy. 

 This principle states that in all useful processes some energy must be degrad- 

 ed and thus lose its ability to do further work. Energetics models incor- 

 porate the second law by requiring heat sinks, or energy degradation flows, on 

 all energy interaction and energy storages. 



279 



