Is variable speed marine air conditioning really worth it?
OK, I admit it. I’m a graphaholic. I can’t turn the pages of a newspaper without studying each and every graph and chart, regardless of the subject matter. I make them, I dream of them, and I get itchy if I don’t pore over a graph or two for a few days. (Today I studied one showing the rate of decline in teenage marriages in Bangladesh since 1970). But I’m also acutely aware that the accuracy of any conclusions gleaned from graphs and charts are totally dependent on the validity of the data they are constructed from. Remember; Garbage in - Garbage out.
I turned to graphs recently in my quest to ascertain whether employing variable speed compressors in self-contained marine air conditioning units (i.e. compressor, blower and other bits and bobs all together on one base) really has any great benefits, especially considering the considerable extra cost and complexity involved.
Compared to a contemporary self-contained unit, a variable speed model requires the addition of a number of special sensors plus an electronically controlled expansion valve, all feeding data into a specialized electronics package that powers a three-phase compressor.
There’s a lot to go wrong there, and no way to jury-rig an override if the electronics suffer premature death from water ingress, lightning, or even just plain ol’ electrickery.
While perusing the promotional material published by what appears to be the sole manufacturer of marine variable speed self-contained air conditioning units, the claim of 50% energy savings leaped out at me right away. But on reading the small print, it states that this is referring to a SEER rating, which is a Seasonal Energy Efficiency Rating for whole-house air conditioning systems, and not applicable to marine self-contained models. I recently replaced a 20+ year old air conditioning system in my house and I wouldn’t be at all surprised if the high-efficiency replacement isn’t using 50% less energy (just please don’t ask why it took me so long to replace it). No, that 50% claim is a generalization referring to domestic systems, and more than just a little misleading when used in discussions on marine units. But if the SEER rating is not relevant for marine units, then what is?
There are two energy ratios we can consider when discussing the efficiency of self-contained marine air conditioning units, and both are Power Out to Power In ratios. In general, the slower a compressor runs, the higher the efficiency ratio number, and the higher the number, the more efficient the system. The COP (Coefficient Of Performance) considers the ratio of cooling watts output to the electrical watts input, while the EER (Energy Efficiency Ratio) compares the amount of cooling Btu’s produced to the watts of energy consumed. Either could be used, but we’ll use the EER here (it’s easier to determine from manufacturer’s published data) and is the result of Btu’s/hour produced divided by Watt-hours consumed. We can ignore the time factor for now, so we’ll use just Btu’s and Watts until we need to consider time later on.
On that same promotional material I mentioned above are some graphs (oh joy!), and these contain all we need to calculate the EER of these units for a given compressor speed. Now, it must be understood that there is no indication of the operating conditions under which this data was collated, so they have to be taken with a pinch of salt. However, we can use them to compare performance at different compressor speeds, as can be seen from the following:
From the performance chart of the 16,000 Btu model:
• Compressor at full speed – 16,000 Btu at 1,000 watts = EER 16
• Compressor at half speed – 8,500 Btu at 450 watts = EER 18.9. That’s a gain in efficiency of just over 18%.
From the performance chart of the 7,000 Btu model:
• Compressor at full speed – 7,000 Btu at 450 watts = EER 15.5
• Compressor at half speed – 5,500 Btu at 300 watts = EER 18.3. Again, a gain (he-he) in efficiency of 18%.
So according to these performance graphs from this one manufacturer, produced from unverified data at unknown operating conditions, these units have an astonishing EER of over 18. Compare that to the most efficient room air conditioners, which are around EER 13, and it gives cause for doubt over the validity of these results. Whether those EER numbers are accurate or not, what the numbers seem to indicate is that the system is 18% more efficient with the compressor operating at half speed compared to full speed.
That efficiency gain looks mighty impressive, especially when compared to performance data I have from one of the most widely used manufacturers of variable speed compressors. In contrast to that of the marine unit manufacturer, this compressor manufacturer’s data has been taken at specific operating conditions that are relevant to water cooled marine applications, and they reveal an efficiency gain of around 8% at half speed. So, is it really possible that a marine air conditioning manufacturer can somehow engineer an additional 10% more efficiency into the completed unit in addition to that of the compressor when running at half speed? Hmm …
I have a variable speed residential system for the upstairs floor of my house, and both the indoor and outdoor fans operate at variable speeds along with the compressor. Blowers on most conventional marine units can be programmed to run at variable speeds automatically, but in a marine application, the outdoor fan is replaced by a sea water pump. Unless something very tricky is done to make the pump operate at variable speed in conjunction with the compressor, this will have a negative impact on power consumption and EER at anything less than full compressor speed.
The sea water pumps we use are quiet and dependable centrifugal models that rely on an impeller spinning at a certain speed to throw the water around a chamber and out of the discharge port. I don’t know what would happen if a centrifugal pump designed for operation on 60 Hz were to be run on, say 30 Hz, but I’m pretty sure you wouldn’t get half-flow, in fact I would be surprised if there was any flow whatever at that speed. In contrast, a diaphragm type pump with positive displacement can be run at any speed and the flow rate will more or less vary accordingly, but these types of pumps are less efficient and less reliable than centrifugal models while being considerably louder.
So, with the above in mind, let’s now re-visit those EER numbers of the 16,000 Btu marine variable speed unit, but this time with the power draw of the sea water pump included. The pump typically recommended for a 16,000 Btu unit is the March LCP-3-CP with a 500 gph flow rating. This pump is rated at 370 watts maximum power consumption, but 200 watts is a more realistic figure in our applications. Without the ability to vary the flow from the pump, we’ll need to operate it at a constant flow rate of 500 gph, as required for 16,000 Btu’s, even though the unit may only be producing half that amount of cooling.
Referring back to the performance graph of the 16,000 Btu variable speed model, let’s look at the power consumption for the self-contained unit plus 200 watts for the sea water pump:
• Compressor at full speed – 16,000 Btu at 1,000 + 200 watts = EER 13.3
• Compressor at half speed – 8,500 Btu at 450 + 200 watts = EER 13.1
That’s a loss in efficiency when running the compressor at half speed compared to full speed.
This shows a very different story when the sea water pump is included in the power consumption figure. But, is it possible that over the course of time the slower speed will end up being more efficient?
Let’s look at our 16,000 Btu unit serving an area with a continuous 8,000 Btu load, as may be the case during night hours or on a dreary day. First with compressor speed fixed at maximum and then at half-speed.
• 16,000 Btu unit fixed at maximum speed running stop-start for one hour – 1,200 watts x 0.5 = 600 watt-hours
• 16,000 Btu unit running at half capacity continuously for one hour – 650 x (8,000/8,500) x 1 = 612 watt-hours
That’s an increase in power consumption when running the compressor at half speed compared to full speed.
Note that the above is a theoretical situation, and there will no doubt be variables in both examples. But, it does illustrate the fact that if the sea water pump is not run at variable capacity in concert with the compressor, any efficiency gains made from running the compressor at low speed will most likely be negated by the inclusion of the power consumption of the sea water pump.
Is there no place for variable speed compressors in marine air condition systems?
Yes, absolutely there is, and that is in the chilled water systems found on larger vessels. Here we have the compressor cooling circulating water instead of air, and which is then pumped to a number of different cooling units in various cabins and spaces around the boat.
The fact that the load is constantly varying as the individual cooling units operate on and off under demand from their thermostats makes it a perfect application for variable speed chiller units. We don’t have variable speed pumps in widespread use on these units at this point, so there are energy losses at lower speeds, but development is ongoing.
There are two important features of variable speed compressors that can be attractive in certain situations.
First up is the very low start-up current required, as the compressor starts off at 0 RPM and gradually winds up to operating speeds. This is in contrast to the in-rush spike of current required by a fixed speed compressor and may allow operation on poor power supplies or undersized generators which cannot deliver an adequate in-rush peak current. However, modern fixed speed compressors have greatly reduced in-rush current demands compared to older designs and can normally start easily on reduced power supplies, even without the addition of a soft-start device.
Second is the fact that a variable speed system does not require a specific power frequency for operation. This means that a variable speed system can work directly from any power supply anywhere in the world, 50Hz or 60Hz, without loss of performance or risk of damage to the compressor. This can also be achieved with fixed speed compressor systems by powering them through an inverter from batteries and having a multi-voltage, multi-Hz battery charger supply the charge current to the battery bank.
I think I’ve answered my own question on this one, and I won’t be rushing out to get variable speed marine self-contained units any time soon. It’s amazing how much information one can extract from graphs and charts, but simply accepting the results without being able to verify where the data came from may lead to some speed bumps ahead.