Efficient Air Conditioning

I've been looking into split ductless air conditioning systems. These units have a compressor mounted outside the home much like conventional central a/c systems. However, the refrigerant is delivered to independent air handling units (i.e. fan coil units) that are strategically placed in the home. An insulated line carries refrigerant supply and return lines, plus a condensate drain tube into the home.

Compared to conventional systems, these units can consume a lot less electricity for the same cooling capacity. This is so for several reasons:

1. Air is not distributed through ducting thereby avoiding a lot energy losses associated with moving air, and the losses associated with poor insulation and poor sealing.
2. The compressor motors often use a permanent magnet rotor which avoids electricity consumption otherwise required to magnetize the rotor.
3. Most important, the compressor motor is generally a variable frequency "inverter" drive that varies the compressor speed based on cooling demand. Conventional systems have to cycle the compressor ON and OFF to maintain the thermostat settings. However, these variable speed drives control the cooling rate proportionally to meet cooling demand at all times. The efficiency advantage here is significant. Based on my research, the reason this works so well is generally not understood. However, it's quite simple. The heat exchangers associated with an a/c system are sized according to the rated capacity of the compressor. Lowering cost is an important factor in sizing these heat exchangers. Therefore, the size of these units are limited partly based on cost. The fact is that increasing the size of these heat exchangers will reduce electricity consumption by reducing the differential pressure across the compressor. Well, the same effect is achieved by keeping the same heat exchanger sizing, but lowering the compressor speed. In effect, the size of the heat exchangers increase relative to the compressor output. Therefore, these a/c units tend to operate very efficiently at a low part load output. The effect is dramatic. The performance of these units are about the same as other modern a/c units when they operate at their rated output, but they increase in efficiency on the order of 50% at a low part load operation. Many units also have programmable thermostats that support variable settings over a 24 hour period. This effect is particularly interesting for those who make use of grid tied solar as the cooling demand is often highest during the day when a solar array is often producing. It's possible to program the a/c unit to a lower thermostat setting for max cooling output when the solar array is producing, then raise setpoint at other times for maintaining a low part load operation and reduce consumption of grid electricity. This approach can even make air conditioning in the off grid setting a viable option by taking advantage of solar array production during the day, but reducing battery discharge at night. Note also that these compressor motors have low starting amperage which is ideal for the off grid setting.
4. There is also the ability to more completely split the cooling. For example, a single fan coil unit in a bedroom might be operated at night rather than cooling the entire home or an entire floor.

Note that most of these units also function as heat pumps to provide space heating very efficiently, and the heating performance is particularly good where winters are mild.

We are interested in alternative cooling systems as well. Your study is of interest, although way out of my league of understanding. (Hubbie would understand more than I would). A few years ago, I found an interesting cooling idea that does use duct work, but, is very simple in that, you just bury 4" PVC, about 6' deep, and about 100' to 150' long. I think it was called "Earth Tubes". We are thinking of implementing this idea. I was about to post about this when I came across your post. Are you familiar with "Earth Tubes"?

If one's summers are arid, evaporative cooling is extremely inexpensive and very DIY friendly

Janet Williams wrote:We are interested in alternative cooling systems as well. Your study is of interest, although way out of my league of understanding. (Hubbie would understand more than I would). A few years ago, I found an interesting cooling idea that does use duct work, but, is very simple in that, you just bury 4" PVC, about 6' deep, and about 100' to 150' long. I think it was called "Earth Tubes". We are thinking of implementing this idea. I was about to post about this when I came across your post. Are you familiar with "Earth Tubes"?

I am familiar with earth tubes. I don't see it as an effective means of cooling, particularly when considering a very hot and humid region where I'll be settling eventually. In my case the annual average temperatures are high, the summer temps are very high, summer humidity is high, and the soil is high in clay which tends to insulate. All this means high humidity, high average soil temps, and poor heat transfer... so in my case earth tubes are a non-starter. If you're in a temperate zone with a dry climate, then perhaps this can be an option. However, in that case you should note David's advise and consider evaporative cooling.

I'll discuss the split ductless a/c units again for clarity (in particular, I'm considering the units with variable frequency drive compressor motors). Don't be intimidated as it's quite simple. I believe these can be ideal for providing air conditioning in the off grid setting. The qualities of the units that lead me to this conclusion include:
1. Low starting amperage. Even small window a/c units often see current spikes on start up that are too high for most power inverters. These split ductless units with variable frequency drive motors see much lower starting current.
2. Let's say you have a large solar array. During 4-6 hours of each summer day the array might produce electricity at a very high rate. A 1.5 ton split ductless a/c unit draws about 1750 watts at full power. If the thermostat is set very low during this period, then the unit will operate at full power while trying to get the air temperature down to the thermostat setting. So, since the solar array is likely generating more than 1750 watts during this period, then the a/c unit will consume this electricity directly (well, via the inverter, of course). Therefore, a lot of battery losses are avoided completely, and the home gets a nice cooling shock that will help carry it into the evening. Now, once the solar array production drops, then the thermostat setting is raised. The system is designed to operate at a very low rate to maintain higher thermostat settings (rather than cycle on and off repeatedly), and it is under these conditions that the efficiency of the system is truly impressive. Therefore, battery discharge is minimized! For example, this 1.5 ton a/c system might draw 1750 watts at full power, but it will draw about 400 watts at 1/2 ton.
3. While a large solar array is necessary to provide a/c in the off grid setting no matter the efficiency of the set up, the split ductless a/c systems also operate in heat pump mode. Therefore, much of the solar electricity production during the winter months need not be wasted. The heat pump mode turns 1 KWh of solar electricity into 3-4 KWh of space heating (assuming relatively mild winters).
4. Many units I've seen can be programmed to change thermostat setting several times over a 24 hour period.

Thanks guys. We live in Mississippi, so its very hot and humid. Sounds like the Earth Tubes would not be a good option. And we're not off grid, but I'm going to explore the evaporative cooling, and let my Hubbie read about this other system.

Janet Williams wrote:Thanks guys. We live in Mississippi, so its very hot and humid. Sounds like the Earth Tubes would not be a good option. And we're not off grid, but I'm going to explore the evaporative cooling, and let my Hubbie read about this other system.

Mississippi and evaporative cooling don't mix... too humid. I have the same problem in east Texas. It seems like my idea might just be best in your case. If you were to remain on grid (which makes strong sense when it's available), then you could build up a grid tied system slowly using the Enphase system. Coupled with the strategy discussed before, you could start to dramatically reduce grid electricity consumption required for air conditioning. I have an interest in all kinds of alternative energy technologies, but this is probably the most practical alternative. Note that the most important things you could do remain such things as insulation, radiant barriers, passive techniques like shading, and higher thermostat settings.

Solar hardware when grid power is available will not likely prove cost effective (maybe with tax credits and especially if you could do the installation yourself), but it would be educational and a lot of fun.

Note: There are micro grid tie inverters available that make it really easy to do this. However, I personally question the quality of these units coming out of China. The Enphase inverters seem to be much higher quality.

Marcos, we looked into the evaporative cooling, and came to the same conclusion. It has to be very low humidity to be effective. However, my Husband did read your report, and says he understands the concept. Whether or not it will be implemented, remains to be seen. There are many unfinished projects around here! Being in the construction field, we try to be efficient homeowners, but there is always something to learn.
I've always dreamed of adobe style walls! Not common here, but I lived in New Mexico for 4 years, and know how effective those thick walls are. Easy to cool and heat. Anyhow, thanks for your response.

Marcos, thats an excellent summary and explanation. Mini-split Heat Pumps MSHP are taking the high performance building industry by storm and I would add that another huge benefit to them is that they work great in heating dominated climates too. Apparently, one of the Mitsubishis can work in temperatures of -12F or so.

These machines are amazing. They can heat AND cool WITH dehumidification and have extremely low energy requirements. This technology is also behind Heat Pump Water Heaters which are arguably more efficient and cost effective than solar thermal, especially in cooling climates. Iam hoping we see this technology eventually paired with refrigerators (the same technology) for extremely energy efficient appliances which can be largely served with PV.

I like how you touched on the tricky balance of PV, building envelope and heating/cooling requirements. I think that too many people are focusing on the more complex technology side of the equation. PV (especially grid tied) and MSHP are awesome and often cost-effective to implement but they need to come AFTER an airtight and continuously insulated building envelope.

Mechanical systems like PV and especially heat pumps are sized based on the efficiency of the envelope. You can reduce this expensive technology (which requires maintenance and replacement) by focusing on excellent blower door test results and high performance insulation. Unlike mechanical systems, the building envelope is quite permanent, requires less maintenance and is very hard to correct and improve once built.

I havent done MSHP yet but would consider it on a home where the homeowners have an open floorplan and are ok with aesthetics and keeping bedroom doors open. They can be tricky to locate to provide comfort to the entire home depending on levels, bedrooms and overall level of openness. I feel that once your are up to three indoor heads or compressors its beginning to get more expensive than a traditional heat pump but things are changing in this area almost as quickly as PV adoption. As noted they make a lot of sense for retrofits.

Evaporative cooling is only for the very driest of climates. Anywhere else, another name for earth tubes could be mold tubes.

I agree that evaporative coolers are most comfortable in humidity levels below 30%. Thick pads are better than thin (reduce dry spots that let in hot air), but there are fewer choices now than before. Some evaporative cooler designs can work efficiently at higher humidity levels but I have not seen them accepted by the market, primarily because they are almost as expensive as a cheap central air conditioner. The evaporative cooler market has suffered tremendously because of restrictive covenants in newer subdivisions that forbid their use.

Has there been any discussion of solar (not PV) absorption air conditioners (similar to gas-fired air-conditioners, which are another option)? I understand there are a few systems around but they have not found acceptance, probably because they use ammonia. I have been following the progress, and lack of it, of a company in Austria, SolarFrost, for several years. A local hvac store owner here said there was a company in San Diego that built similar systems.

Andrew Parker wrote:Has there been any discussion of solar (not PV) absorption air conditioners (similar to gas-fired air-conditioners, which are another option)? I understand there are a few systems around but they have not found acceptance, probably because they use ammonia. I have been following the progress, and lack of it, of a company in Austria, SolarFrost, for several years. A local hvac store owner here said there was a company in San Diego that built similar systems.

Hello Andrew. I made a mention of absorption and adsorption chillers in other posts (both biomass fueled and solar). The company that is making the greatest advancements on this front is SorTech of Germany (see sortech.de). Their systems use water as the refrigerant and silica gel (or zeolite) as the adsorbent. I like the technology, but objectively I have to go with a highly efficient vapor compression cycle for reasons of simplicity, effectiveness, and low cost. The coefficient of performance of these chillers are about 0.6-0.7. However, there are thermal losses seen during the heat harvesting process that is substantial. A good system could see an overall (thermal) performance of 0.4-0.5 when these thermal losses are considered. This is similar to a PV powered vapor compression system, especially when considering that these adsorption units can consume electricity at a fairly high rate if the heat source driving the cycle is low (cooling towers are required, and additional air moving equipment and even water consumption is required). In the final analysis a highly efficient PV powered vapor compression system combined with excellent insulation, radiant barriers, and other passive techniques like shading, etc., is hard to beat (especially for a modest residential setting).

Just making a note. The efficiency gains achieved when these variable speed compressors operate at a low part load is not so high as I first considered (at least not for most units I've been able to examine). Still, it is an important effect. It's really the combination of all the factors I listed that lead to higher efficiency. In particular, doing away with duct work and the high power fan motors require to force air through the system, along with losses due to leaks and imperfect insulation, makes significant gains in efficiency possible. Not having to cool an entire home also contributes greatly to lower energy consumption (i.e. split cooling/heating). I have seen that split central HVAC systems are becoming more popular with newer two story homes often having the ability to heat/cool the lower OR upper floor. Also, I've seen central a/c systems that use two compressors (one small and one large) to lessen compressor cycling, improve dehumidification, and just plain lower energy consumption. Still, it's the ductless feature of these systems that I now see as making the most significant difference.

An alternative to earth tubes:

http://mb-soft.com/solar/ac77.html