Keeping Cool in Sustainable Ways
Air conditioning is almost 70% of our peak power demand.
This year, Texans and Californians have once again been asked to use less electricity during heat waves—and grid managers have prepared plans for rolling outages to prevent public power grids from failing. Even in New England, power demand peaks in the summer due to residential and commercial air conditioning. What steps can we take to keep air conditioners humming for everyone as summer draws to a close?
Our electricity grids are most stressed during summer heat waves.
Thanks for joining us as we explore ways to increase our energy parsimony, stretching how long every kilowatt hour can keep us cool. (If you’d like a refresher on what energy parsimony or kilowatt hours are, check out “First Steps to Endless Energy.”) Many of the steps we can take now to stay cool will help us stay warm when seasons change; early fall is a great time to take action to save energy.
This week we’ll explore steps for energy conservation, sacrificing some comfort, care, or convenience to save energy (and often money, too):
Acclimate to warmer temperatures.
Move to a cooler location; don’t crank up the air conditioning.
Shade windows.
Use fans or dehumidifiers instead of air conditioners.
Turn off lights and other appliances.
Set back thermostats on air conditioners.
We’ll also explore steps for energy efficiency to stay cool, saving energy without sacrificing comfort, care, or convenience (but requiring an up-front investment):
Landscape with plants that provide shade and evaporative cooling.
Install solar awnings over southern exposures.
Improve air sealing and insulation, especially in basements and attics.
Install heat-reflecting roofing material or solar panels (or both).
Use zoning to make the best use of cooling resources indoors.
Use evaporative coolers (if suitable for your local climate).
Install high-efficiency heat pumps or air conditioning systems.
Install an energy recovery ventilation system.
Weekly Poll: A Fan of AC?
Help! How can I live without AC?
If you’ve decided not to use air conditioning (or simply don’t have AC), your body will naturally acclimate to warmer temperatures—up to a point. Sweating is our healthy response to being hot. The drier the air, the more effectively we can cool by sweating.
Most of us can safely tolerate a “web bulb” (100% relative humidity) temperature up to about 95. Factoring in air temperature and humidity, the following are equivalent to the upper limit of human endurance:
95 degree air temperature at 100% humidity
100 degree air temperature at 82% humidity
105 degree air temperature at 67% humidity
110 degree air temperature at 55% humidity
120 degree air temperature at 37% humidity
You can take these steps to increase your natural ability to tolerate heat:
Wear loose-fitting clothes that cover your body but allow you to sweat.
Drink often to stay hydrated. If you’re feeling thirsty, take a sip of water.
Take a nap outside in the shade during the afternoon.
Get up early to take advantage of naturally cooler weather in the morning.
Stay out of direct sun.
Move more slowly and take more frequent breaks than you do in cooler weather.
At night, sleep outdoors or next to an open window for breezes.
You or your landlord can take many steps to improve the energy performance of your home or office building:
Plant deciduous trees (ones that have broad leaves) and bushes in appropriate locations to provide summer shade.
Install window treatments (shades or solar films) to keep sunlight from coming in.
Install exterior solar awnings over windows and doors at the correct angles to provide mid-day shade in the summer but allow winter rays to reach indoors. The noon sun is higher in the sky in summer and lower in winter.
Improve air sealing and insulation so your building can retain cool air longer. You want to control where and when air flows in and out.
Install heat-reflecting roofing material or solar panels (or both). Solar panels not only can keep your building cooler, but can also provide electricity on hot summer days to help satisfy peak power demands.
Normal living activities (including cooking, showering, and simply breathing) increase indoor air humidity levels. You can take several steps to reduce indoor air humidity without air conditioning:
Keep roofs, walls, and foundations in good repair to prevent rain and groundwater from getting inside.
Cook outdoors or prepare meals that don’t require boiling water.
If you do cook inside, run an exhaust fan in your kitchen.
Run exhaust fans in your bathroom during and for ten minutes after showering.
Hang laundry to dry outside.
Use desiccants like zeolite or silica gel. These are materials that absorb moisture. They work best in small, enclosed spaces (such as clothes drawers or closets). You can dry out and reuse desiccant packets by heating them up in direct sun. A solar oven is especially effective for this.
Run a dehumidifier. Like desiccants, portable dehumidifiers are best in enclosed spaces like a bedroom or office. If you work from a home office, you may find that keeping doors and windows closed and running a dehumidifier is more energy efficient than a window air conditioner.
Moving air also helps us stay cool. You can catch cross breezes by opening windows on opposite sides of your home and ensuring air can flow unimpeded. Even on a still weather day, you can create a convective breeze by opening windows at different heights. For example, opening a window at the bottom of a flight of stairs and a window at the top will permit a flow of slightly cooler air coming in downstairs and warmer air going out upstairs.
Fans move air but also produce waste heat from their motors. It’s most effective to run fans only when you are in the room. Turn on ceiling and box fans when you enter a room; turn them off when you leave. For this type of fan, if you’re not there to enjoy the moving air, you’re wasting energy and money. Set ceiling fans to blow air down—you should feel the flow of air when you stand below the fan. For box fans, during the day, set them to blow out a side window that is not in direct sun. Keep shades closed in any windows that get direct sun. At night, you can turn box fans around to suck outdoor air into your home, blowing cooler air onto you while you sleep.
Attic fans and whole house fans operate differently: run them when outdoor temperatures are cooler than indoor. Attic fans are installed in the roof or sidewall of an attic space, and are designed to exhaust hot air and replace it with cooler outdoor air. Whole house fans are installed in the floor of an attic, and are designed to suck air up and out of your living space into your attic.
Make sure you have adequate make up ventilation—that is, a way for as much fresh air to come in as your fans are pushing out. For attic fans, install a passive vent (i.e., one that opens automatically when the fan runs) large enough to allow just as much air into the attic as the fan pushes out.
You can get solar-powered attic fans that run automatically; this works best if your attic is sealed off from the rest of your house so there’s no risk of accidentally depressurizing your living area. In contrast, whole house fans should not run automatically; instead, you should operate them manually, typically in the evening or at night. Check that outdoor air temperature is lower than indoors, open a few windows several inches, run the fan until all your indoor air has been replaced with cooler, drier outdoor air, then turn off the fan and close windows.
Help! How can I lower my AC bills?
If you decide you need air conditioning, you can take several steps to get the most value out of every dollar you spend on it:
Upgrade your lighting and appliances to be more energy efficient. For example, incandescent lighting turns 90% of the electricity it uses into heat, not light, whereas LED lighting produces almost no waste heat. Focus on improving your lighting, your water heater, and your refrigerator for maximum savings.
Install a heat pump hot water heater. These appliances absorb ambient warmth to heat up water, cooling and drying the air around the water heater.
Install an energy-recovery ventilator (ERV). These provide fresh air but exchange moisture and heat so that incoming air in the summer becomes cooler and drier. With an ERV, you can keep your windows tightly closed and the AC running, but still enjoy very high-quality fresh air brought in from outdoors.
If you use window air conditioning units:
Consider using a dehumidifier instead. You may find that keeping windows shaded and tightly shut and running fans and a dehumidifier is a better option.
If possible, put your portable AC unit in a window that does not get direct sun. If you have no choice but to put the unit in a window that gets direct sun, install an awning to shade the AC unit and the window.
Make sure that all gaps around the window AC unit are tightly sealed. All the air you condition will eventually leak out—a hole the size of a quarter lets this happen almost twice as fast as a hole the size of a dime.
Use the highest temperature (lowest power) setting that keeps you comfortable.
Have reasonable expectations: a window unit works best to keep just one room cool, not an entire apartment or floor of a house.
If you have central AC:
Set your thermostat to 78 degrees when people are inside.
Manually set back your thermostat to 88 degrees when you’re not home—or upgrade to a programmable or a smart thermostat. A programmable thermostat does exactly what you tell it (e.g., “set the temperature to 78 at 7 a.m., set it to 88 at 10 a.m.”, etc.) while a smart thermostat can figure out how to achieve your goals (e.g., “make sure the house is cool when I wake up, let it warm up while I’m at work, and cool it down again by the time I get home from work”).
Use zones to keep just the rooms you need cool and not your entire house. Close doors to rooms that you are not using.
Help! We’re thinking of installing heat pumps; how well do they work for AC?
Heat pumps are reversible refrigeration systems, typically “ductless mini-splits.” The “mini” part of the name refers to the small physical size compared to conventional AC units, and “split” refers to the fact that the evaporator “head” is inside while the compressor unit is outside. There are also full-sized ducted central heat pump systems.
We’ll cover heat pumps in more depth in a future action guide, but for now, just know that you’ll get the same or better air conditioning performance from a heat pump system with a reversible valve as you’d get from an AC-only system with a one-way valve. Heat pumps and air conditioners use the same rating system:
Cooling power is traditionally expressed in “tons” or “British thermal units per hour” (BTU/hr). These ratings can be converted to kilowatts (kW):
one cooling ton = 12,000 BTU/hr = 3,516.85 watts = 3.51683 kWThe efficiency of an air conditioner or a heat pump in cooling mode is measured by its Seasonal Energy Efficiency Ratio (SEER): its cooling output over a typical cooling season, divided by the energy it uses. In 2023, the Department of Energy updated the test conditions for determining this ratio, so cooling equipment now receives a SEER2 rating. The higher the number, the better; most equipment sold today has a SEER2 rating between 13 and 21.
Help! We’re replacing our AC; what should we look for in a new AC system?
Upgrading your air conditioning system is a great opportunity to upgrade your heating system, too. Modern heat pump systems can provide both cooling and heating using the same equipment, unlike older technology that had separate AC and fuel-burning furnaces. So that’s the first thing: it probably makes sense to put in a new air conditioner that has a reversing valve—that’s what a heat pump is.
Heat pumps use compressors (pumps) for cooling. We’ll explore how that works before considering more energy-efficient cooling technologies such as evaporative cooling and desiccant-enhanced evaporative cooling.
Since 1834, vapor compression has been the tried-and-true technology for refrigeration. You’ll find a sealed tube, compressor, condenser, expansion valve, and evaporator in most refrigerators, freezers, air conditioners, and heat pumps. A pump compresses gasified refrigerant in a sealed tube, sending the refrigerant gas through a condenser, where it gives off heat and condenses into liquid. After that, pressure forces the liquid through an expansion valve and into an evaporator, where the liquid absorbs heat and evaporates back to gas on its way through the compressor again.
If you install a vapor compression heat pump system, handling condensate is one of the most important considerations. Air next to the evaporator cools down and can’t hold as much moisture. On the inside walls of the evaporator chambers, very cold liquid refrigerant is warming and turning into gas, but on the outside of the evaporator, water is condensing out of the chilled air. A condensate drain collects this water and takes it away. These condensate drains are notorious for growing mildew and becoming clogged, so plan ahead to be able to inspect and clean them.
The refrigerant in a vapor compression system can be problematic. Many refrigerants (such as Freon) are now banned because they deplete our ozone layer or act as powerful greenhouse gases. It’s hard to know exactly which refrigerants will be allowed in the future, but some people feel carbon dioxide (R744) is a good bet. Whichever refrigerant is in your system, try to ensure that none of it leaks out.
More efficient technologies for air conditioning take advantage of the fact that we can lower air temperature by evaporating water. Traditional evaporative coolers (also called “swamp coolers”) are effective ways to lower air temperatures in hot, dry climates. Rather than using electricity to pump refrigerant in a loop, evaporative coolers trickle water droplets through an incoming airstream. These water droplets absorb large amounts of heat.
Evaporative coolers cost about one-half as much to install as central air conditioners and use about one-quarter as much energy. However, they require more frequent maintenance than refrigerated air conditioners and they're only suitable for areas with low humidity.
—Evaporative Coolers, United States Department of Energy
Efforts are now underway to enhance evaporative coolers by adding desiccants (air drying materials) to them. This simple idea—first dry out the air using a desiccant material that absorbs moisture, then send dried air through an evaporative process to cool it down—could make evaporative coolers practical for hot, humid climates.
Desiccant-enhanced evaporative cooling isn’t the only refrigeration technology in the labs: solid-state refrigeration may eliminate the need for refrigerant gases and may one day greatly improve the reliability and efficiency of cooling (and heating) systems. But for now, vapor compression and evaporative coolers are widely available.
What’s Still Ahead on the Pathway…
Earlier this year, we explored the pathway to sustainable movement; now we’re exploring the related pathway to sustainable energy. What are the best ways to save, use, and make energy? Stay with us on the journey to sustainability as we take action to have a positive impact on the world.
References and Further Reading
California Residents Urged to Limit Air-Con Use During Heat Wave, Newsweek
Texans asked to reduce energy use again as ongoing heat wave strains power grid, The Texas Tribune
Conserve energy to prevent rotating outages, Pacific Gas & Electric
New England’s Electricity Use, ISO New England
First Steps to Endless Energy, Fred Horch
Stay Cool, National Institutes of Health
The emergence of heat and humidity too severe for human tolerance, ScienceAdvances
Wet Bulb Calculator, Omni Calculator
How Can Buildings Reduce Window Heat? The 5 Best Window Films for Heat Reduction, Window Film Depot
Cool Roofing Shingles, Owens Corning Roofing
What is Zeolite?, Ida-Ore
What is silica gel and why do I find little packets of it in everything I buy?, How Stuff Works
2023 Buyers Guide: 5 Best Solar Ovens, EcoWatch
Dehumidifiers, ENERGY STAR
How Dehumidifiers Work, How Stuff Works
Heat Pump Water Heaters, ENERGY STAR
Energy recovery ventilation, Wikipedia
What is the Best Temperature for AC?, Carrier
Heat Pumps, Consumer Reports
What’s a Good SEER2 Rating?, Trane
This is the HVAC tech that will replace vapor compression, GlobalSpec
Freon, Wikipedia
Basic Ozone Layer Science, US Environmental Protection Agency
Refrigerants Are the Worst Greenhouse Gases You’ve Never Heard Of. Here's What You Can Do, KQED
Why CO2 is the Most Promising Refrigerant in the Cooling Industry, ISA Interchange
Evaporative Coolers, US Department of Energy
These moisture-sucking materials could transform air conditioning, MIT Technology Review