Evaluating SiC efficiency and payback in warm and cold climates

By Pranjal Srivasta and Sarah Magargee, Wolfspeed

In this article, we explore how two European cities with differing climates can benefit from designing heating and cooling systems with silicon carbide power devices instead of traditional Si-IGBTs (insulated gate bipolar transistors).

First, let’s explore how some common residential heat pump systems (10-15kW three-phase inverter) perform in Istanbul’s warm climate versus Oslo’s cold climate. Figure 1 illustrates seasonal inverter efficiency in systems with silicon carbide vs Si IGBT and reveal efficiency improvements from 2% in the cold climate of Oslo to upwards of 4% in Istanbul.

Figure 1: Seasonal inverter efficiency in systems with SiC vs Si-IGBT

At first glance, the efficiency improvements may seem counterintuitive. The graphs presented indicate that while the efficiency gain with SiC in colder climates is lower than in warmer climates, the overall actual energy savings are greater in colder regions.

This phenomenon can be attributed to the load dynamics of heat pumps. In warmer climates like Istanbul, the heat pump operates at lower loads, meaning that while the efficiency improvement is significant – around 4% – the overall energy demand is lower (Figure 2). Consequently, the total energy savings achieved with SiC inverters are not as pronounced.

Figure 2: Load dynamics impact annual savings

In colder climates, the load on heat pumps is considerably higher. Although the efficiency improvement of SiC over IGBT is about 2% – lower than the 4% seen in warmer climates – the higher energy demand in colder regions results in substantial overall energy savings. This underscores the importance of considering both efficiency and load when evaluating the performance of heat pump systems.

The high/low load differential and impact on efficiency and energy savings can be seen in a similarly high-power system optimised for cost and size.

Figure 3 illustrates a 25kW air compressor inverter switching at 8kHz and the efficiency improvements seen with a 30A SiC 1200 V 6-pack power module. This power module is smaller in current rating and physical dimensions then the 1200V IGBT modules it is compared to (50 and 100A rated).

Figure 3: Illustrating the efficiency improvements with SiC at partial and peak loads despite a smaller heat-sink, in a 25KW peak load 3-phase air compressor inverter switching at 8kHz

The graph on the right shows the 30A rated SiC and the 50A rated Si IGBT reaching similar junction temperatures at peak loads, despite lower current rated SiC MOSFET with a 77% smaller heat sink. Efficiency improvement would be greater if similar sized heat-sinks were used in systems not optimised for size or cost.

The inverter efficiency improvement with SiC MOSFETs at partial loads is higher than at peak loads due to linear transfer characteristics of SiC vs. non-linear characteristics of Si IGBT with a knee voltage, leading to significantly higher conduction losses at partial loads (Figure 4).

Figure 4: Under lower loads the SiC MOFSET conduction losses can be nearly half that of Si-IGBT

SiC also has lower switching losses than IGBT, making them ideal to reduce heat pump audible noise in fast switching air compressor inverters. Figure 5 demonstrates how Si IGBTs reach thermal overdrive at partial loads in the same 25kW inverter switched at 16kHz instead of 8kHz, while the SiC based inverter exhibited higher thermal stability and efficiency improvement with a 40% smaller heat sink. This could result in a quieter, more optimised and efficient heat pump.

Figure 6: Variable cost analysis in a 25kW Air Compressor System

Energy efficiency and system lifetime payback in warm vs cool climates

The correlation between climate and efficiency/energy savings is also seen in system lifetime payback calculations. A thermally stable residential SiC powered 25kW inverter switching at 16kHz with a mix of peak and partial load conditions in our cold Norway climate could realise an estimated ¢4,586 savings over the systems’ lifetime when compared to a Si IGBT powered inverter switching at 8kHz. Additional savings could also be realised if the SiC powered inverter were switching at higher frequencies.

This is clearly highlighted (Figure 6) in the variable cost difference analysis of the two systems and including the active front end, which connects directly to the grid supply. Despite the SiC module being about a twice as expensive as the higher current rated Si IGBT modules, the overall BOM cost is expected to be only about 15% higher which would be paid back in quick time due to higher energy savings, particularly at partial loads.

Conclusion

The takeaway from these findings is that while SiC inverters demonstrate greater efficiency improvements in warmer climates, the real benefits in terms of energy savings are realised in colder climates. This is due to the higher operational demands placed on heat pumps in these regions, leading to faster payback periods for systems utilising SiC technology.

Focusing on specific cities like Istanbul and Oslo reveals a compelling narrative. While Istanbul shows a higher efficiency improvement with SiC, the overall energy savings in Oslo are greater due to the higher energy demand. This highlights the critical role that load conditions play in determining the effectiveness of different
inverter technologies.

In summary, the analysis underscores the importance of designing heat pump systems with SiC technology to maximise energy savings, operational efficiency and payback, regardless of climate.

This article originally appeared in the April'25 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.