Q&A about heat pump design - CAREL
Q&A about heat pump design - CAREL
This is my third and last Q&A post about heat pumps. After a general overview of this interesting application in the Q&A and a second post focusing on efficiency, I have collected a few questions on some of the most interesting design aspects.
Designing heat pumps means analysing their components, such as compressors and expansion valves, selecting the right number and type of sensors and defining the most convenient layout with an eye to carbon footprint and future maintenance, among other matters.
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How to develop a high-efficiency heat pump?
High-efficiency heat pumps currently rely on variable capacity DC inverter compressors, electronic expansion valves, electronic fans and pumps. They all fit permanent magnet motors for improved efficiency compared to asynchronous motors and can modulate capacity to match the load in real time.
Modulation is the key factor to better energy efficiency compared to fixed capacity devices of any kind, which are just on or off.
Well-designed heat exchangers are energy-saving components as well. “Well designed” is the key here because the plate heat exchangers for water and finned coils in air sources have been the norm for decades. The R&D for both water and finned coil technologies is now focused on good refrigerant distribution and this has become even more important considering the modulation capabilities of the other components mentioned above. Poor distribution leads to poor performance and a waste of the heat exchange surface.
Finally, controllers are very important because they are the “brains” of the heat pump. They must optimise the control of all devices together, such as the compressor safety envelope, the fan/pump vs compressor energy consumption balance, and most of the time they manage the energy-saving defrost and hot water production features.
What roles will sensors play in the heat pump market?
Heat pumps, as any other air conditioning and refrigeration application, have always relied on temperature and pressure sensors.
In the Far East, especially in Japan, manufacturers produce very few models in very large numbers. In this way, they design heat pumps by mapping them in all conditions, define the control logics, and then remove most of the sensors. Incidentally, they work with a few temperature sensors.
Instead, the market in western countries is fragmented with plenty of customization needs. In the current and future residential market, there will be a lot of players, at least compared to the few operating in Japan, with many different models and layouts. Mapping is not suitable.
Furthermore, there will be a great need for reliability and simplicity of use to compete with the current market reference (gas boilers). Heat pumps are a key application for decarbonization, which also means that governments will need to “certify” their efforts to achieve the goal by measuring the efficiency increase offered by this technology.
As a result, more than just temperature and pressure sensors will be needed. The need for measuring the efficiency of millions of heat pumps on the field will drive the use of cost-effective energy meters and heating capacity meters (water flow meters and precise temperature sensors). Additionally, the growing interest in the use of Artificial Intelligence and IoT technologies based on big data will require connectivity, more temperature and pressure sensors for maximizing heat pump reliability, and information on weather forecasts, which may come from sensors or the Internet.
What about cascading, work in parallel and upgrading existing heat pump units?
Cascade heat pumps are more suitable for the industrial market, where the low and high temperatures of the sources can vary greatly. Multiple refrigerants appropriately chosen according to the temperature range are used in this case.
I remember having observed the use of cascade heat pumps in the residential Korean market right after the Fukushima disaster. This terrifying event forced all neighbouring countries to massively reduce energy consumption. Korean heating systems were based on electric boilers at that time and there was a great need of short-term conversion to heat pumps. High temperature lift heat pumps were needed to comply with the existing typical water distribution systems and the Korean external temperature profiles and a R-410a / R134a cascade solution was found. Nowadays, I think R-290 heat pumps can work with a good COP down to -25°C of the external temperature and up to 80°C of water delivery temperature so that cascade systems are rarely needed in this market.
On the other hand, the concept of modular design is not a bad idea. We have years of experience of multiple R-290 circuits in the refrigeration field to reduce the charge per circuit and comply with the safety standards. So, I think a modular design, with multiple circuits in the same unit or multiple units, can match heating capacities above 12-15kW.
Finally, concerning the upgrade of existing units we have no direct experience beside the modular design mentioned above. I would just like to stress that this approach fits better the industrial market.
What is the carbon footprint of a heat pump/chiller system for AC per nominal kW compared to a direct Air/Air packaged unit?
Comparing the carbon footprint of these two systems is hard, at least without precise specs and measurements.
In general, I can say that direct expansion units are always more efficient than air-to-water units simply because they do not need an extra "refrigerant to water" heat exchange. I’m focusing on efficiency because the carbon footprint in HVAC applications is mainly dependent on energy consumption.
However, air-to-water heat pumps are suitable for various plant layouts, including the most efficient underfloor heating and cooling that requires low compressor ratios to work. For example, direct expansion heating usually works with 40°C air outlet temperature and 45-50°C refrigerant condensing temperature. Underfloor heating, on the other hand, can work at 30°C water temperature, with 35°C refrigerant condensing temperature.
For sure, this comparison is not fair, but it is worth mentioning.
In terms of carbon footprint, refrigerant choice and charge are involved as well. Air-to-air packaged units and air-to-water heat pumps are mostly the same in this regard. We can assume that the shift towards R-290 will be quick in both cases.
Can you provide a short troubleshooting guide for electronic expansion valves for heat pumps?
The troubleshooting of EEVs does not depend on the application type (cooling, heating, refrigeration, air conditioning). It involves three aspects: mechanical, electrical and control issues.
The most common mechanical issue of EEVs is step loss. A stepper motor, assuming we are talking about this kind of EEVs, can lose steps during movement. With a proper closed loop electronic control, the negative impact of this issue is limited because the valve position is adjusted according to sensor feedback. Furthermore, controllers have many step recovery functions, such as zero/100% position extra steps or resync when regulation stops.
Leakages are more critical issues. They are diagnosed by checking the evaporator pressure, which can drop slightly or significantly depending on the extents of the leakage, and blockages. A pressure switch alarm is very likely to trigger when an EEV is blocked. Blockages are more frequent in full closing position, and their root causes are dirt, pressure shocks and ice formation due to water/moisture in the circuit, mechanical or electrical failures.
From the electrical point of view, wiring errors are the most common issue. An incorrectly wired EEV will not move or move in the wrong direction (closing instead of opening). Wire or coil failures, sometimes due to water infiltrations, are less frequent.
In any case of presumed de-sync between assumed and real EEV position, manually moving the valve using magnets is a good way to troubleshoot the issue.
Finally, control issues are assumed when the valve regulation is unstable (valve position and sensor values swing up and down around a reference setpoint). Most controllers use PID algorithms to define the valve movement vs sensors measurements (most commonly superheating, temperature or pressure). PID parameter adjustment cannot be explained briefly in a few words. I simply suggest firstly pinpointing whether the instability is caused by the EEV regulation. This can be determined by setting the EEV in manual positioning and stopping it in a defined position (i.e. 50%). If instability is still present, it probably comes from a different source, such as the fan/pump or compressor regulation, for instance. If it is caused by the EEV, most likely the regulation is too “reactive”, and it should be slowed down by reducing the proportional gain (P) and/or increasing the integral time (I) and/or reducing the derivative time (D).
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