International. The 46% improvement of SEPR (Seasonal Energy Performance) obtained by replacing a conventional overheating control with a control based on steam quality technology. A 100 kW chiller powered by R717 by direct expansion is tested in Karlsruhe as a bachelor's and master's project.
Most chillers and heat pumps built as direct expansion plants use conventional control based on the temperature and pressure of the gas leaving the evaporator. An alternative to the conventional approach is to use a vapor quality sensor, which can measure the liquid content in the gas coming out of the evaporator. By controlling the expansion valve based on the quality of the steam, overheating can be eliminated and heat transfer improved, which improves the COP (coefficient of performance).
How the two control systems were tested
It can be difficult to test and compare the two methods of control in plants that often have a unique design. Therefore, the best result is obtained in a test environment. At Hochschule Karlsruhe, the student has built and tested a chiller with two different control systems. The cooler can be operated with both control systems and it is easy to switch between the two systems and measure the difference.
The cooler is designed with two plate heat exchangers that function as an evaporator and condenser, both heated/cooled by liquid. The load is 4 kg of ammonia (R717) and the capacity is 100 kW. The temperature settings are made to match the 2016/2281 regulation following the EU ecodesign directive, making it comparable to other chillers. The output of the chiller is 12/7 ° C according to the standard.
The table shows a reasonable improvement of 21% at full load, but a huge saving of 51% on partial load. Testing is performed according to Ecodesign guidelines.
The result: big improvements in SEPR and COP
The cooler is tested at different capacities that match SEPR specifications, both operating with overheating control and with control based on steam quality, and the differences are significant. SEPR can be seen as a specific COP value, where the limits reflect temperature variation over a year, making it a much better measure.
Meet Ecodesign requirements
The requirements to comply with the ecodesign directive for a smaller water-cooled chiller is SEPR> 6.5 until 2021 and 7 from 2012. This means that the tested chiller meets eco-design requirements when using steam quality control, but not when using the conventional control system.
When steam quality control is used, annual energy consumption is reduced by 28 MWh equal to €3350 based on the average EU electricity price for non-consumers (€0.12/kWh). Over a 10-year period, the savings are of the same magnitude as the cost price of the full cooler.
Source: HB Products.
