The advantages of system-level simulations compared to thermodynamic cycle calculations

Introduction

Thermodynamic cycle calculations are widely used to design heat pump units, giving insight into their performance at design point. Most compressor selection software now has these built-in, making the cycle calculation simpler. However, because these calculations are based on a lot of assumptions, they often don't match real-world operation data / test data. System-level simulations of heat pump can help bridge this gap.

Thermodynamic cycle calculation

The figure below is an example of a cycle calculation from Frascold compressor’s selection software. To perform a cycle calculation, we need to define inputs such as:

• Condensing temperature
• Evaporating temperature
• Subcooling degree
• Superheat degree

Once these inputs are defined, the cycle can be illustrated on a Pressure-Enthalpy diagram. From this diagram, we can obtain valuable information, including:

• Coefficient of Performance (COP)
• Heating capacity
• Cooling capacity
• Compressor power consumption

These results provide essential insights for evaluating the system configuration of a heat pump.

However, thermodynamic cycle calculations have some limitations:

• These calculations do not account for heat transfer in the condenser and evaporator. They simply assume that the condensing and evaporating temperatures will be perfectly reached and remain constant. In real-world heat pump operations, the complex heat transfer within the heat exchanger means that condensing and evaporating temperatures are always affected by operating conditions, such as changes in ambient temperature and air flow rate through the heat exchanger. Therefore, cycle calculations cannot accurately capture the characteristics of the heat pump when these operating conditions change.

So, a series of laboratory test of a heat pump are necessary to evaluate the system performance under various operational conditions.

System-level simulation of heat pump

What is system-level simulation of heat pump?

System-level simulation of a heat pump involves creating detailed, physics-based models of its components, such as the heat exchanger, compressor, and valve. For instance, modeling heat exchangers with two-phase flow requires applying mass, momentum, and energy balance equations to accurately represent the heat transfer and fluid flow mechanisms. For compressors and valves, semi-empirical models are typically used, which are developed based on combination of physical laws and regression analysis of test data or operational data.

How to use system-level simulations for heat pump?

To use system-level simulations for a heat pump, follow these steps:

• Determine the specific types of heat exchangers and compressors to be used in the system. For example, you might select plate heat exchangers from SWEP B26Hx14 and SWEP V200T for the condenser and evaporator, and a compressor from Dorin CD300H.
• With the identified components, develop a heat pump model within the simulation environment. Based on the specific configuration of the heat pump, set up the model to simulate the entire system.

The figure below illustrates the most classic heat pump configuration.

To conduct a system-level simulation of a heat pump, the following key inputs are required:

• Condenser side secondary fluid flow rate
• Condenser side secondary fluid inlet temperature / outlet temperature
• Evaporator side secondary fluid flow rate
• Evaporator side secondary fluid inlet temperature / outlet temperature
• Compressor frequency

Once the simulation is run with these inputs, the following performance metrics of the heat pump can be obtained:

• Coefficient of Performance (COP)
• Heating capacity
• Cooling capacity
• Power consumption of compressor
• Temperature, mass flow rate, pressure in the cycle

What is the difference between system-level simulations and cycle calculations?

Inputs used:

• System-level simulations use operational conditions as inputs, such as the inlet flow rate and inlet temperature on the condenser's secondary side.
• Cycle calculations, on the other hand, use condensing and evaporating temperatures as inputs, which can always fluctuate during real-world operation.

Heat transfer and fluid flow:

• System-level simulations account for detailed heat transfer and fluid flow within heat exchangers and also consider compressor performance. This allows for accurate performance evaluation of the heat pump under varying operational conditions.
• Cycle calculations do not capture these detailed mechanisms, leading to less accurate performance predictions when operational conditions change.

Dynamic simulation:

• System-level simulations can provide both steady-state and dynamic results. Dynamic simulations are particularly useful for representing transient characteristics, aiding in the evaluation of control strategies of heat pump.
• Cycle calculations typically only provide steady-state results and do not capture transient behavior.

Benefit of system-level simulations of heat pump

The table below is an excerpt from the EN 14511 test standard for brine-to-water heat pumps, showing only the low temperature test conditions. As indicated in the table, the variables that can be adjusted during testing are the secondary side inlet temperature, outlet temperature, and flow rate. By changing these parameters, we can evaluate whether the heat pump's performance under these part-load conditions meets expectations.

As previously described, system-level simulations use the same inputs as test conditions. After developing a heat pump model in the simulation environment, a limited number of tests can be conducted to validate the model's accuracy by comparing the simulation results with actual test data. Once validated, the heat pump model can be employed for a large number of additional tests within the simulation environment. This approach reduces laboratory testing time and product development costs, also reduces the development of prototypes.

Summary

Thermodynamic cycle calculations have been widely used in the heat pump industry to provide important information on heat pump performance at the design point. However, due to their assumptions, these calculations do not account for heat transfer within heat exchangers and cannot accurately represent heat pump performance under part-load conditions.

In contrast, system-level simulations of heat pumps use a physics-based approach, considering detailed heat transfer, fluid flow within heat exchangers, and compressor performance. This method is highly effective for evaluating heat pump performance under variable operational conditions. By potentially replacing some laboratory tests, system-level simulations can significantly reduce product development costs and time.