Followup on Two-Stage Evaporative Cooling

Two-Stage Evaporative Cooler from OASys

Two-Stage Evaporative Cooler from OASys

[This follows up on the previous post, This is Not My Beautiful House. Taken from “The Globally Optimized House: AC,” written August 11, 2010.]

Most of my work (that paid) has been some form of global optimization, from optimizing parallel programs and schemes to automatically optimize parallel programs, to optimizing plan layouts in subdivision, to managing investments considering all tax and estate consequences and adverse events.

The new house in Palm Springs is no exception. Built in 2003, by the standards of the time it’s well-insulated and efficient. At that time, electricity cost about 12 cents per kWh; currently the highest tiered rate (which we will easily reach using AC) is above 30c/kWh. The pricing scheme is hugely complicated and unpredictably micromanaged by an incompetent state legislature and regulators, and discriminates against large families as well as wasteful users. It does, however, result in such high marginal costs for larger houses that solar power, as currently subsidized, is more than competitive for the highest-tier rates.

The former owners were doing what most people do, shutting off AC in most of the house and using it sparingly in the areas they actually live in; this kept their bills down to $400/month or so, averaged year-round. In our usage with less than half the house cooled to 79-84 F, the power bill for the month of July would have been about $600 if we had been there full-time.

Heating and cooling buildings is a complex global optimization problem. First note that what we want to optimize is not the temperature as shown by the thermostats (there are 6), but human comfort; if there is no one at home, there is no need to control anything (though furnishings can suffer from excess heat or extremely high or low humidity.) We want the house to know how many people are in what areas to determine how hard to work to condition the air. Also, comfort depends on many factors; temperature, humidity, moving air, radiation temperature (easily noticed in winter when it can feel cold at an apparently comfortable temperature because cold walls or windows soak up thermal radiation.) I am trying to give up hot coffee in the summer since a cup can make me uncomfortably warm for an hour when an iced coffee would not have.

The general problem of cooling can be approached using a variety of sources and sinks available in the environment. Groundwater, for example, is often at a reliable low temperature and can be used for cooling, either through a heat exchanger or indirectly by using it as a heat sink for a conventional AC compressor. In the desert, evaporating water is an energy-efficient cooling method. [BTW. don’t let anyone tell you that there’s a water shortage and that evaporative cooling will make it worse; fossil fuel power plants use more water to produce the additional power needed for conventional AC than evaporative cooling uses. One source comments that use of evaporative cooling for a typical house results in additional water usage of about a shower per day, implying it would strain water resources; but since Southwestern homes typically use many times that amount on landscaping irrigation, and generation of additional electricity for conventional AC would use as much, additional water use is not really an issue.]

In a climate with wide swings in outdoor temperatures, simple ventilation and storage can provide most of the heating and cooling in many seasons; a whole-house fan and the house’s heat storage capacity obviates the need for costly AC much of the time, but it requires constant monitoring of conditions and control of ventilation, and is most effective when outside conditions in the next 12-24 hours are known. A smart person can handle this, and until now, most whole-house fans have been controlled by a simple switch and the strategic opening and closing of windows.

There’s huge room for improvement in the technology of AC. Manufacturers have improved conventional compression-cycle refrigeration AC a great deal since energy prices started to climb; SEERs (Seasonal Energy Efficiency Ratios) have climbed from 10 to 14-16 in two decades, cutting power used by a third.

But the original form of cooling, used since ancient times in dry climates, passes hot, dry air over water to cool and humidify it; this is now called evaporative cooling. In its simplest form, a fan blows hot dry air over a medium soaked with water. Evaporating water cools the medium, which cools the air passing over it, which meanwhile picks up some moisture. In a desert climate, outside air at midday can be at 110 F and less than 10% humidity; the wet-bulb temperature (the temperature a thermometer registers when it is cooled by a soaked medium after air is blown over it) can be below 50 F. These simple evaporative coolers, known as “swamp coolers,” were widely used in the desert Southwest until cheap AC units and the bad reputation of swamp coolers for high maintenance and growth of microorganisms in the medium led to their replacement by conventional refrigerative AC. There are times of year, also, when the humidity levels rise enough to reduce a swamp cooler’s effectiveness, so that for some weeks of the year, the output air from a swamp cooler is uncomfortably warm and humid.

More recent development of two-stage evaporative coolers resolves most of those issues. By using outside air in a first stage to cool one side of the medium, but exhausting the now-moister air and then drawing in more outside air to pass over the already-cooled moist medium on the other side, the air can be cooled more with less addition of humidity. This is not as efficient as a one-stage swamp cooler under ideal conditions for their use (because it uses more fan power to move more air), but works in a much wider range of conditions, and can cool a space for about 1/3 the cost of conventional AC (SEERs of 40 vs AC’s 14). The fungus and microbial issues of old-style swamp coolers have been dealt with by a variety of automated purging and cleaning methods, though some disinfection and annual maintenance is still a good idea.

Many other factors influence comfort and can be tweaked to improve it; our house is not ideally oriented, with its long axis north-south and prime living areas facing west. The best designs for passive solar heating and cooling have the house laid out east-west, with a large southern overhang sized to allow in winter sun and keep out summer sun. We spend a lot of time opening and closing 15-20 shades on the east and west sides of the house as solar incidence changes. A set of automated shades with a smart controller which can respond to conditions would handle that for us…. [ultimately a window technology which can be controllably tuned to allow in or block light and heat will simplify this problem, but while this has been an area of research for some time, no cheap and practical windows of this type exist.]

[Updating with a bit from Green Building Advisor, “Saving Energy With an Evaporative Cooler:”]

Whatever happened to the OASys?

Many articles on evaporative coolers mention the OASys, an energy-efficient appliance that was developed by engineers at the Davis Energy Group in Davis, California. Unfortunately, the OASys is no longer available.

For a while, the OASys was being manufactured by Speakman. After a while the production of the unit was moved to a factory in India. Because of poor sales, however, manufacturing was discontinued. The remaining units were shipped to a warehouse in Nevada, where a warehouse fire destroyed the entire inventory.

The end of a historical era

A few years ago I had a conversation about evaporative coolers with John Proctor, the president of Proctor Engineering Group in San Rafael, California. Proctor, a nationally known air conditioning expert, told me that evaporative coolers deserve wider use. “The problems with direct evaporative coolers are overblown,” notes Proctor. “I lived in an evaporatively cooled home in Colorado for many years and was extremely happy with it. It worked well. I’m befuddled by the fact that more people don’t use evaporative coolers.”

Like most GBA readers, Proctor is a strong advocate for the use of energy-efficient HVAC equipment, so his love of evaporative coolers isn’t too surprising. Although energy-efficiency advocates have been singing the praises of evaporative coolers for years, the tide seems to have turned away from these devices. Evaporative coolers are fading away.

A reporter for The Arizona Republic, Ryan Randazzo, described the trend in a 2010 article titled “Once-Common Evaporative Coolers Are Disappearing from Phoenix-Area Homes.” Randazzo wrote, “Now the metal boxes atop homes are rare, done in by a combination of cheap and increasingly energy-efficient air-conditioning and the time and expense of maintaining the coolers. Arizonans steadily have moved away from using the sometimes noisy, always drippy evaporative coolers, even though they may reduce energy bills.”

According to Randazzo, almost every Phoenix home had an evaporative cooler in 1940. In 1984, nearly half of all Phoenix residents still had one. By 2010, however, less than 10% of Phoenix homeowners had an evaporative cooler. “Most residents who still use them are either extremely cost-conscious, handy at fixing the units, or both,” Randazzo reported.

Randazzo continued, “New housing developments are limiting coolers’ use on roofs. And people are just happy to use an air-conditioner that rarely needs repairs vs. a cooler that needs rooftop service at least twice annually.”

Randazzo interviewed Mike Donley, president of Donley Service Center. Donley explained that “most people just tired of climbing on the roof in the spring to clean and activate their cooler, and getting up there again in the fall to clean it and seal it off. … ‘Coolers are a do-it-yourself project,’ Donley said. … ‘I hate to say this, but if you are going to [pay to] have us service it twice a year, you are better off buying a high-efficiency AC system,’ Donley said.”

So unless a manufacturer steps forward with a two-stage cooler that doesn’t require maintenance at all (which is possible; an automated device could clean itself, as ours did in part), people will think of it as too much trouble and move to use less efficient, overly drying AC.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s