Solar Airconditioning

Salam Aidilfitri~

How is everyone's weekend, especially to all the Muslims around the world who is celebrating the Holy Month of Ramadhan? I'm sure everyone must be busy preparing all the new clothes, refurbish the kampung house or even getting all the materials for ketupat, rendang, nasi lemak, etc. I did enjoy the local delicacies in Malaysia. They sure know how to make great hospitality to visitors.

These few days had been very hot prior to the Muslim celebration. It seems that always the hot weather will happen before any mass celebration such as Chinese New Year and Deepavali. Maybe it's a blessing for those who celebrate so they can go anywhere without rain coats or umbrellas. During my work in my office, as I was looking outside, there are many commercial buildings that are still occupied with white collar people. I was thinking: instead of using energy efficient cooling system, why not use a solar air-conditioning that harnesses the power of the Sun during the office hour? The theory seems practical because the cool temperature is only needed during the day, not at night when people had gone home. That way, we do not need to spent any electricity energy at all. Wow, talk about another fantastic idea.

What Is Solar Air-Cond?

Solar air conditioning refers to any air conditioning (cooling) system that uses solar power. This can be done through passive solar, solar thermal energy conversion and photovoltaic conversion (sunlight to electricity). Photovoltaics can provide the power for any type of electrically powered cooling be it conventional compressor-based or adsorption/absorption-based, though the most common implementation is with compressors. For small residential and small commercial cooling (less than 5 MWh/a) PV-powered cooling has been the most frequently implemented solar cooling technology. The reason for this is debated, but commonly suggested reasons include incentive structuring, lack of residential-sized equipment for other solar-cooling technologies, the advent of more efficient electrical coolers, or ease of installation compared to other solar-cooling technologies (like radiant cooling).

Another Thing Called Passive Solar?

In this type of cooling solar thermal energy is not used directly to create a cold environment or drive any direct cooling processes. Instead, solar building design aims at slowing the rate of heat transfer into a building in the summer, and improving the removal of unwanted heat. It involves a good understanding of the mechanisms of heat transfer: heat conduction, convective heat transfer, and thermal radiation, the latter primarily from the sun.

For example, a sign of poor thermal design is an attic that gets hotter in summer than the peak outside air temperature. This can be significantly reduced or eliminated with a cool roof or a green roof. Passive solar cooling is much easier to achieve in new construction than by adapting existing buildings. There are many design specifics involved in passive solar cooling. It is a primary element of designing a zero energy building in a hot climate.

What Is the Cost?

Since PV cooling's cost effectiveness depends largely on the cooling equipment and given the poor efficiencies in electrical cooling methods until recently it has not been cost effective without subsidies. Using more efficient electrical cooling methods and allowing longer payback schedules is changing that scenario.

For example, a 100,000 BTU U.S. Energy Star rated air conditioner with a high seasonal energy efficiency ratio (SEER) of 14 requires around 7 kW of electric power for full cooling output on a hot day. This would require over a 20 kW solar photovoltaic electricity generation system with storage.

A solar-tracking 7 kW photovoltaic system would probably have an installed price well over RM80,000 (with PV equipment prices currently falling at roughly 17% per year). Infrastructure, wiring, mounting, and NEC code costs may add up to an additional cost; for instance a 3120 watt solar panel grid tie system has a panel cost of RM4.12/watt peak, but still costs ~RM8.85/watt hour peak. Other systems of different capacity cost even more, let alone battery backup systems, which cost even more.

A more efficient air conditioning system would require a smaller, less-expensive photovoltaic system. A high-quality geothermal heat pump installation can have a SEER in the range of ±20. A 100,000 BTU SEER 20 air conditioner would require less than 5 kW while operating.

There are new non-compressor-based electrical air conditioning systems with a SEER above 20 coming on the market. A less-expensive partial-power photovoltaic system can reduce (but not eliminate) the monthly amount of electricity purchased from the power grid for air conditioning (and other uses).

Ups & Downs

Should you consider going with one of these solar-centric units, instead of some other A/C-only brand of mini split? It depends on the end user's lifestyle and expectations. The DC-only version, in my view, would have a fairly narrow appeal, as it's currently designed. Because it's not grid-tied, it will only run in daylight hours, when cloud cover permits, and it does require batteries, even for daylight-only operation. This type of unit might be suited to a detached home office or studio, but unless you want to invest significantly in battery storage, would not perform during the critical bedtime hours, when a lot of people like to keep their space cool.

The solar hybrid system, however, does not require batteries. Instead it's tied in to 415 volt grid power, but does not require a special contract with the utility because it does not backfeed. It taps into the grid only when the sun stops shining. Like the DC-only unit, it requires about 885 watts of power for peak cooling, so PV arrays can be fairly modest, if you're only running one unit, which is caple of cooling about 750 sq. ft., in a well insulated building. Sizing the PV array to supply "just enough" electricity could be what pushes these units into the mainstream. Someone who wants to dabble in solar can put up three panels and start saving on electricity right away.

PROS

• Incentive programs - The cost of setting up solar air conditioning can be very high. But a family or business that opts for solar powered ACs can reduce long term electricity costs that make the initial expense worthwhile. Additionally, both the federal government and many state governments offer incentive plans to solar energy users. 
• Increase home resale value - In addition to stabilizing soaring energy costs, solar energy panels are considered an investment. Many homeowners find that solar features add considerable value to their home, and make their homes more desirable in a competitive real estate market.

CONS

• Weather-dependant - Solar energy panels for cooling have one obvious requirement - the sun. If you choose solar AC, you could have a significant problem on days where the skies aren't clear enough for the panels to soak up the sun's energy.
• Tracking the sun - For optimal efficiency, solar energy panels need to constantly be directed towards the sun.
• Cleaning - In addition to tracking, solar panels need to be oriented and cleaned.
• Expensive batteries - Aside from the issue of power storage, prospective solar energy consumers need to consider the costly battery reserve. Batteries are expensive, require maintenance, and don't last very long (unless you plan to use in the daytime only)
• High set-up cost - Aside from battery costs and replacement panel costs, the initial set-up cost for solar energy systems can be considerably high. In the long run the system is worth it, particularly in areas where there is constant sun and a constant need for cooling.

So What's The Verdict?

I hope I'm not being too hard on this new technology. As an affordable way to bring net-zero HVAC to homes all over the world,it could be a real game changer. The biggest advance, of course, is in the efficiency of mini-splits. A few years ago SEER 35 seemed like an impossible benchmark. But it's the super-efficiency of the equipment that makes the case for solar power so compelling.

What's left for both the DC-only units and the hybrids is to work out the problem of solar panel power variability. Sure, the off-grid units can work around this with batteries. But  for the hybrids, if the power goes out, they stop working completely, even on a sunny day. The ability to deliver at least some cooling directly from the sun would improve their attractiveness to buyers.