Frequent Questions
Solar Air Conditioning

The refrigerant fluid in an air conditioner transports the heat energy it absorbs from inside a building through an outdoor heat exchanger that radiates this heat into the air. However, heat only flows from warmer to cooler, so an air conditioner compressor must raise the temperature of the refrigerant fluid until the refrigerant is significantly warmer than the outside air.

A compressor’s efficiency can be improved—and its operating cost reduced—by reducing the difference between the compressor’s inlet and outlet temperatures. In our system, a high-performance solar collector assists the compressor by raising the refrigerant fluid’s temperature before it enters the compressor, so less work is required from the compressor to reach the desired temperature of refrigerant leaving the compressor.

A solar thermal collector is perfect for this job because the amount of solar energy available is greatest precisely when the compressor has to work the hardest (think hot sunny day).

Yes, absolutely. In fact, the key advantage of an evacuated tube solar collector is that the heat pipe tube containing the heat transfer fluid is surrounded by a vacuum, so heat loss to the surrounding air by conduction and convection is eliminated. This characteristic is particularly useful during cold winter days, when there is lots of sunshine but the air temperature is much lower than the desired operating temperature of the heat transfer fluid inside the solar collector. Our system uses a reverse cycle heat pump, so it can also lower your heating costs during the winter.

The biggest reason is that our heat pump has a special two-speed compressor that consumes much less energy during periods of low cooling demand—and during periods of higher demand when available heat energy from the solar collector allows the compressor to run at the lower speed.

There is also a second reason why a two-speed compressor is important for our solar-assisted system. Without the lower operating speed, the solar-assisted system might not run long enough to provide humidity control. Low speed operation allows humidity control without significantly increasing energy consumption.

No. This approach is frankly just a gimmick to sell conventional high-efficiency heat pumps with the added sex appeal of solar electric power. Here’s what is really going on: A small solar photovoltaic (electric) panel powers the condenser coil fan motor during the day. (This is the exhaust fan on the outside unit.) However, the power consumed by a condenser coil fan motor typically represents only six to seven percent of the power consumed by the entire air conditioning system.

For example, the compressor for a three-ton heat pump might draw about 18–20 amps of electric current during normal operation. A 3/4 HP blower motor in the air handler draws another six to eight amps. The condenser coil fan motor is usually only 1/4 HP and draws less than two amps. So allowing for air conditioner operation at night and during cloudy weather, it is difficult to see how this type of system could cut your air conditioning costs by more than five percent.

The brochures and sales presentations for these “solar air conditioners” usually include the suggestion that you can increase the number of solar electric panels in the future, eventually powering your air conditioning system entirely with solar energy. Don’t fall for it. Using solar electric panels to power your home’s central air conditioning system would be just about the worst renewable energy investment you could possibly make.

Solar energy can only achieve dramatic savings in an air conditioner’s operating cost if it helps the compressor run less often. That’s exactly what our system does.

Simple math. Only about 10 percent of the solar energy that strikes a solar photovoltaic (electric) panel is converted into net electrical power that can be used by an alternating current motor. An evacuated tube solar collector, by contrast, converts 40 to 60 percent of the solar energy that strikes it into thermal energy that can reduce the compressor motor’s workload.

Add to this the fact that solar photovoltaic panels are more expensive per square foot of collector surface area than evacuated tube solar collectors, and the idea of using solar photovoltaic panels to power an air conditioner starts to look pretty silly.

A reverse cycle heat pump is just an air conditioner with an extra valve. This valve allows the refrigerant to flow in the opposite direction for cold weather heating. Just think of it as the system working backwards: absorbing heat into the refrigerant from the outside air, using the compressor to raise the refrigerant’s temperature even more, and then transferring the heat from the refrigerant to the indoor air as it passes through the coil in the air handler.

During normal cooling operation, the refrigerant fluid in your air conditioning system absorbs heat from indoors and then—with the compressor’s help—dumps it outdoors through the condenser coil, which operates very much like a car radiator. The condenser coil—like a car radiator—is just a heat excahnger. It’s a simple matter to also run the heat-carrying refrigerant through a second heat exchanger, only this one dumps heat into your hot water tank instead of the outdoor air.

In addition to potentially providing all your hot water during the hottest months of the year, a waste heat recovery unit can also improve your air conditioning system’s efficiency. Part of the improvement is achieved because water has greater thermal conductivity than air, and the balance of the improvement occurs because tap water is usually a bit cooler than the outside air temperature when the air conditioning system is running the most.

No. The compressor has a pressure switch that shuts the motor off when pressurized refrigerant leaving the compressor reaches the pressure associated with the desired condenser coil inlet temperature. So the compressor will do less work and consume less electrical energy—and the pressure switch will shut the compressor off sooner—if refrigerant enters the compressor at a higher temperature.

Yes, but refrigerant circulating through the solar module after dark doesn’t matter. Here’s why:

Remember that the temperature of refrigerant entering the solar collector heat exchanger from the air handler evaporator coil is only 50°F or so. The lowest nighttime air temperatures during the summer months in Florida are usually in the mid-70s, and even during the spring and fall months, if the weather requires air conditioning, it is very unlikely that the nighttime air temperature will fall below the 60s.

In other words, there is never a time during the cooling season when the refrigerant is cooler than—or even close to the temperature of—the outside air. Since heat always flows from warmer to cooler, any heat exchange with the solar collector will always raise the temperature of the refrigerant.

Also, we have found that the solar collector can actually remain quite hot during the hours following sunset.

No, refrigerant does not circulate through the solar collector module’s glass tubes. The refrigerant only passes through a metal heat exchanger in the horizontal manifold that runs across the top of the solar collector module. Each evacuated tube unit “plugs into” this manifold. If a glass tube breaks, it is simply removed from the manifold and replaced.

You could, but the energy savings would be dramatically less and the solar collectors would require extra equipment: a control system and circulating pump, an expansion tank and a separate heat exchanger. These other types of solar collectors lose too much heat energy to the surrounding air as their operating temperature rise: Flat plate solar collectors cannot consistently deliver fluid temperatures in excess of 130°F under normal operating flow rates, and unglazed plastic solar collectors are limited to a normal operating range of 95°F to 120°F.

An evacuated tube heat pipe solar collector module doesn’t need all this extra complexity, and it can consistently produce temperatures of 150°F to 180°F under normal operating flow rates, performance that matches up much better to the inlet temperature requirements of air conditioner condenser coils.

The most obvious thing everyone will tell you is to clean the filter in your air handler regularly. Two less obvious things are to make sure all your registers (air vents) are open and to leave as many room doors open as much as possible. Closed registers and doors increase the pressure the blower motor in your air handler has to work against. This may require the blower motor to run a bit longer and may accelerate development of leaks in your air ducts. In short, the system may lose efficiency faster.

No, this is a common misconception. You can pretty much assume that you will save money any time you raise your thermostat during the cooling season. An air conditioning system takes about 10 minutes or so to reach peak efficiency every time it cycles on. Fewer start and stop cycles are easier on the system and produce higher average operating efficiency.

When you return home at the end of the day and the unit is forced to run longer to cool the house down, two good things happen: First, the system works at peak efficiency because it runs continuously with fewer starts and stops while cooling the house down. And second, the condenser unit can dump heat into the air more efficiently because the outdoor air temperature falls as the sun sets.

Well, you could invest in a programmable thermostat that will lower the termperature setting to a predetermined level, an hour or two before you usually return home. But the discomfort you feel has less to do with the air temperature and more to do with heat that has built up in the attic all day.

Your attic insulation doesn’t block heat transfer, it just slows it down. By the end of a hot summer afternoon, this insulation has absorbed a great deal of heat from the attic air, which it spends the early evening hours radiating into the living spaces below. Much of the discomfort you feel is infrared radiation (heat energy) from the ceiling striking your skin. While lower air temperature can offset this sensation, a properly installed Attic Shield™ in your attic can eliminate it.

Well, you should absolutely make sure you have the most efficient air conditioning system possible if you’re trying to keep a house that cool during the summer. On the other hand, your air conditioner may be just fine. Here’s why:

No matter how low you set your thermostat, a properly sized system typically cannot cool a house by more than 20 degrees below the outside air temperature during the hottest summer days. This means that if the outside air temperature is 95°F, then 75°F is about the lowest temperature that a properly sized and functioning system will cool the house down to.

Also, you wouldn’t want a bigger air conditioning system in order to get you that extra few degrees during a heat wave. Oversized air conditioning systems are a bad idea, especially in Florida. An air conditioning system needs to run long enough to remove moisture from the indoor air, and this is a special concern in Florida, which has very high humidity. An oversized system also tends to cycle on and off too frequently, which results in excess wear and tear on the equipment and less efficient cooling.

Uh, no. It does not.