When speaking about the performances of a two-phase flow system like a heat pump, dynamic system simulation is a great tool. Simcenter Amesim has a set of built-in demonstrators about refrigerant Rankine cycles and heat pumps. A first step to create such a simulation model is to predesign the desired cycle and check its performances.
Let us go through a use case of a heat pump with the following key characteristics:
- Condensing temperature: -10°C
- Subcooling at condenser outlet: -5°C
- Evaporating temperature: 45°C
- Superheat at evaporator outlet: 10°C
With this information, the cycle is fully defined (given the compression is isentropic), and we can calculate the coefficient of performance (COP) of the system.
The workflow is derived in 3 steps:
Watch this video to see how it works:
Further down the details of the workflow in 3 steps
New features of Simcenter Amesim 2019.1 ease this process by taking advantage of the refrigerant fluid properties built-in database. A clear workflow and a direct visualization of the fluid properties are available in a dedicated user interface that displays the cycle and its characteristics in the steady state.
As a first step, it is important to know what the condensing and evaporating pressures are. Using the two-phase fluid properties app of Simcenter Amesim, these are very easily obtained:
- Condensing pressure: P=27.27barA
- Evaporating pressure: P=5.73barA
Let us open the thermodynamic cycle analysis app from the app space directly and create the cycle. The thing I like about using the app space is that you do not even need to search for a component in the libraries.
#1 Context: selection of the fluid to be used and display of its characteristics. The full database of Simcenter Amesim Two-phase Flow library is available and the equation of state can be chosen.
#2 Settings: Definition of the thermodynamic transformations and visualization of the cycle on the P-h and the T-s diagrams. For each transformation inlet and outlet points, you can reuse an existing point or define it. In the screenshot below, the evaporation is linked to the previous expansion and the outlet state is defined with a zero pressure drop and a target superheat of 5°C.
#3 Post processing: Key characteristics of the transformations are shown. By specifying which transformation brings the required work (compression) and the useful heat transfer (evaporator), the COP is automatically calculated at 3.4825.
Now, this COP corresponds to an ideal cycle as several assumptions were taken along the way:
- zero pressure drop in both the condenser and the evaporator,
- isenthalpic expansion with no heat exchange along the way,
- a compression stage with an efficiency of 1 (isentropic).
Let us challenge this last assumption as it most impacts the COP value. Here is the COP value for various isentropic efficiencies of the compression: