Energy Conversion
Most technical systems transform energy from one form to another. Energy converters include, for example, the following components:
Regardless of whether fuel is converted to heat, electricity to heat, or heat to cold, for example, the principles explained below apply to all modeled units.
Design Parameters of the Nominal Condition
For the layout (design) of a technical system, the nominal size must be specified.
When converting one form of energy into another (e.g., in the components Hot Water Boiler, Steam Turbine, and Power-to-heat), the nominal state is defined by the three quantities
The corresponding equation is:
\( \begin{equation} \begin{aligned} P_{in\ nominal} \cdot \eta_{nominal}= P_{out\ nominal} \end{aligned}\end{equation}\).
Two of three quantities from this equation must be given, and the third quantity is calculated. Therefore, the components offer the option of selecting which sizes are specified (see following slider).
The design values that are not specified but calculated are shown in the Output data. If, for example, the useful and the supplied Nominal capacity are specified, the Nominal efficiency can be seen in the Output data under Design parameters (see following figure).
If a component converts several forms of energy, several efficiency equations must be given, e.g., for a CHP:
\( \begin{equation} \begin{aligned} P_{in\ nominal} \cdot \eta_{1\ nominal}= P_{out1\ nominal} \end{aligned}\end{equation}\) und
\( \begin{equation} \begin{aligned} P_{in\ nominal} \cdot \eta_{2\ nominal}= P_{out2\ nominal} \end{aligned}\end{equation}\).
Here \( \begin{equation} \begin{aligned} P_{out1} \end{aligned}\end{equation}\) corresponds to the electric current with the associated current efficiency and \( \begin{equation} \begin{aligned} P_{out2} \end{aligned}\end{equation}\) to the heat with the associated heat conversion efficiency.
The conventions for heat pumps and chillers differ slightly from those just described. Although the efficiency can be defined physically as above, a different key figure is usually used.
For heat pumps, the Coefficient of performance (COP) is considered instead of the efficiency. The COP does not take into account the conversion of all supplied energy into cooling energy, but only the conversion of the supplied electrical energy, because it is assumed that the supplied thermal energy is “free”. For comparison, you can see the efficiency of a heat pump here:
\( \begin{equation} \begin{aligned} \dot{Q}_{heat} = \eta \cdot (P_{el}+\dot Q_{reheat}) \end{aligned}\end{equation}\) and here is the COP:
\( \begin{equation} \begin{aligned} \dot{Q}_{heat} = COP \cdot P_{el} \end{aligned}\end{equation}\).
In contrast to a physical efficiency, the COP can also have values greater than 1.
Refrigerating machines behave in a similar way: Refrigeration machines with mechanical drive energy (cooling towers, compression refrigeration systems) work with an Energy Efficiency Ratio (EER). EER considers only the electrical energy as expenditure and, for example, not the re-cooling capacity, because it is mostly provided by the environment:
\( \begin{equation} \begin{aligned} \dot{Q}_{cool} = EER \cdot P_{el} \end{aligned}\end{equation}\).
The Energy Efficiency Ratio EER can also have values greater than 1.
The term heat ratio \( \zeta\) is used for thermal drive energy (in adsorption heat pumps). The re-cooling capacity is not included in the heat ratio:
\( \begin{equation} \begin{aligned} \dot{Q}_{cool} = \zeta \cdot \dot{Q}_{heat} \end{aligned}\end{equation}\).
The following description of the part load behavior using efficiency \( \eta \), can be transferred to the conventions described here for chillers and heat pumps.
Design Parameters of the Part Load Behavior
There are three possibilities for specifying the part load behavior. The default setting is a Constant efficiency. Alternatively, the part load behavior can be specified by absolute or relative values which generate a characteristic curve (see following figure).
Constant Efficiency at Part Load
If Constant efficiency is selected, an efficiency is specified which is unchanged under part load (see figure above). Then applies at every operating point, not only the nominal operating point:
Supplied power \( \begin{equation} \begin{aligned}\cdot \end{aligned}\end{equation}\)nominal efficiency = effective power:
\( \begin{equation} \begin{aligned} P_{in} \cdot \eta_{nominal}= P_{out} \end{aligned}\end{equation}\).
In addition to the efficiency, either the Maximum Power Consumption is specified and from this the Maximum Power Supply in the nominal state is determined or the Maximum Power Supply is specified and from this the Maximum Power Consumption is calculated.
If Characteristic Curve is selected, the part load behavior is specified piecewise linearly by values in the table: absolute or relative.
Enter Characteristic Curve (absolute values)
When selecting Enter Characteristic Curve (absolute values), the relationship between the effective power \( \begin{equation} \begin{aligned} P_{out} \end{aligned}\end{equation}\) (e.g., in the Power-to-Gas component: the Fuel Power) and the supplied power \( \begin{equation} \begin{aligned} P_{in} \end{aligned}\end{equation}\) (e.g., in the Power-to-Gas component: the electrical Power Consumption) is established via a Characteristic Curve \( \begin{equation} \begin{aligned} f_{abs}\end{aligned}\end{equation}\) with the following formula:
\( \begin{equation} \begin{aligned} P_{out} = f_{abs} (P_{in}) \end{aligned}\end{equation}\).
Because absolute power ratings are specified with this characteristic curve (figure above) an additional specification of the efficiencies is not necessary.
Enter Characteristic Curve (relative to nominal power)
When selecting Enter Characteristic Curve (relative to nominal power), the relationship between the effective power \( \begin{equation} \begin{aligned} P_{out} \end{aligned}\end{equation}\) (e.g., Fuel Power) and the supplied power \( \begin{equation} \begin{aligned} P_{in} \end{aligned}\end{equation}\) (e.g., Electrical Power) is established via a characteristic curve \( \begin{equation} \begin{aligned} f_{rel} \end{aligned}\end{equation}\). This characteristic curve represents the relationship between the relative part loads of the various forms of energy. It is calculated by the following formula:
\( \begin{equation} \begin{aligned} \frac{P_{out}}{P_{out\ nominal}} = f_{rel} \left ( \frac{P_{in}}{P_{in\ nominal}} \right)\end{aligned}\end{equation}\).
In addition to the values in the table, the Maximum Power Supply or the Maximum Power Consumption \( \begin{equation} \begin{aligned} P_{nominal} \end{aligned}\end{equation}\) and the Nominal Efficiency \( \begin{equation} \begin{aligned} \eta_{nominal}\end{aligned}\end{equation}\) must be specified (see following figure).
Minimum Part Load
A minimum part load can be specified in the input field (see the following figure).
When specifying the part load behavior with an absolute or relative characteristic curve, this must be completed to the point (0;0).
In addition, there is the option to modulate below minimum part load, with which the component can be switched on and off several times within a simulation time step (clocking operation) in order to achieve part loads that are on average below the minimum part load. This reduces the operating time compared to normal part load operation.
A component with 100 kW nominal thermal power and a minimum part load of 50 % can generate 25 kW on average in one hour by generating 100 kW for 15 minutes and being switched off for 45 minutes. The component therefore has only 15 operating minutes within an hour.
If Modulating Operation Below Minimum Part Load is not permitted, the component cannot be operated below the Minimum Part Load.
Own Demand
Some components (Hot Water Boiler, Steam Generator) have an additional Own consumption form under the Input data. Here you can enter the amount of energy required by the component itself that is not directly converted into the useful energy to be generated, for example electricity for the control system. This can cause additional pins, for example electricity pins, to appear on the component.
Output Data: Operating Behavior
The Output data contain information about the operating behavior. As a rule, the actual outputs and the current efficiency are displayed.
Besides the Operating hours, the Full load hours are shown. The Full load hours take into account the amount of energy produced annually in relation to the rated output. By default they are given in hours per year (h/a), the unit can be changed by selecting from the drop-down list, e.g., in percent.
In some components, the Number of starts of the component in the whole simulation period is given.
In addition, current conversion efficiencies and total conversion efficiencies are output. The current conversion efficiencies–e.g., the efficiency of a CHP (see following figure) or a boiler, the coefficient of performance (COP) of a heat pump and the Energy Efficiency Ratio (EER) of a chiller–are calculated for each time step. An overall conversion efficiency, e.g., the annual average efficiency of a CHP (see following figure) or a boiler, or the Seasonal Energy Efficiency Ratio (SEER) of a chiller or a heat pump, refers to a whole year.
