Cooling System

July 26, 2010
Through most of July we did fairly well keeping the house cool by opening windows at night and closing them up during the day. As long as we could get night-time cooling, the house stayed pretty comfortable even through some very hot weather, due the thermal mass in the floor slab and the super-insulated shell, plus overhangs over the south-facing windows to keep out direct sunlight in the summer. And then allergy season struck with a vengeance! Because our ventilation system has a very high efficiency filter on the air intake, we can keep the indoor air quality quite high and levels of pollen low by keeping the house closed up and running the ventilation system for fresh air. But unfortunately this is at odds with opening all the windows at night to cool down the house – we had to choose between keeping the air clean or keeping the house cool. So we decided to build a cooling system.

A typical whole-house central air conditioner has the capacity to remove anywhere from 3 to 20 kilowatts of heat from a house, and it consumes from 1 to 7 kilowatts of electrical power to do so. So even the smallest whole-house air conditioner takes about 1000 watts of electrical power to operate, and if it runs an average of 8 hours per day it will consume 8 kilowatt-hours of electricity per day. That’s approximately what we have been consuming to run the entire house, so adding a small whole-house air conditioner would double our energy consumption! Because of our extreme level of insulation and other strategies to avoid unwanted heat gain, we could probably get by with just a window-size air conditioner but even that would add at least 50% to our energy consumption. Fortunately, we have a better alternative.

Our cistern contains about 12,000 gallons of rainwater that we have collected, and counting the mass of the cistern’s concrete walls and floor that’s roughly 100,000 pounds of thermal mass. It is currently at a nice cool temperature of 64 degrees F. It has not only a lot of mass, but also a lot of surface area in contact with the earth 10 feet below ground level so it should remain cool even if we add some heat to it. This is a perfect place to dump unwanted heat from the house; all we need is a way to transfer heat from the house into the cistern.

To make a cooling system, we ran two lengths of 5/8″ PEX tubing from the mechanical room to the cistern. At the cistern they are connected to a 60-foot coil of 1/2″ copper tubing. The junctions use soldered-on connectors that join the PEX to the copper.

The heat exchanger coil is placed in the bottom of the cistern where the water is coolest. The water is actually quite clean, but it has some dust floating on the surface that makes it look dirty in the photo below. You can see the copper coil under 8 feet of water. It would probably work a little better to stretch it out more, so it could spread heat over a larger volume of water, but there isn’t much we can do about that without going for a swim.

Once the coil was in place, the other ends of the PEX tubing were connected to the fill/drain hose connections in the hydronic heating (now cooling) system. When the hydronic pumps are running the water circulates down through this heat exchanger in the cistern and then out through the loops in the floor slabs.

We started the system up and let it run for about an hour to stabilize, and then made some measurements to see how it was working. The two pumps are circulating about 2.2 gallons of water per minute on their “medium” speed setting. Water coming out of the floor slabs is at 72 degrees F, which is about the same as the surface temperature of the floors. After it circulates down through the copper coil in the cistern it’s coming back at 65 degrees, a 7 degree temperature drop. This lets us calculate how much heat we’re transferring out of the house and into the cistern: about 7700 BTU/hr or 2,200 watts. This is equivalent to a small window air conditioner, and less than even a small whole-house air conditioning unit if it were running all the time. But we don’t need much cooling capacity because the house doesn’t gain much heat, so it should be enough to keep it cool. We’ll need to run it for a few weeks to see how well it performs in hot weather.

Air conditioning units are given an Energy Efficiency Rating (EER), a number that indicates how efficiently they transfer heat out of a building. The EER is equal to the heat removed in BTU/hr divided by the electricity consumed in watts.  An efficient unit that meets EnergyStar guidelines has an EER of at least 11.5, and the most efficient ground-source heat pump systems have an EER of around 20. At the moment our system is removing 7700 BTU/hr of heat from the house and the pumps are consuming 160 watts, so our EER is 48, which is four times as efficient as an EnergyStar air conditioner and well over twice as efficient as even the best heat pumps available! Unfortunately this is not a steady-state condition, because we are using the 100,000 pounds of thermal mass in the cistern as a heat sink and it is slowly warming up. We won’t be able to measure the true efficiency of the system until it reaches steady-state, where the heat is transferred down into the earth below the cistern at the same rate that we remove it from the floor slab. An educated guess is that the system’s true efficiency will be about half what we’re seeing right now, which is still an EER of 24 and more efficient than any commercially-available cooling system.

Although the numbers above may sound impressive, this is just the beginning of an experiment and we have yet to see how well the system performs in terms of maintaining a comfortable interior temperature during hot weather. We think it will work for us, but only because we need very little cooling capacity in the first place due to the superinsulated structure and our other strategies to avoid unwanted heat gain. And it is only cost effective because we had already constructed the cistern for another purpose and we already had the hydronic system to circulate water through the floors – the added cost of the cooling loop was less than $200 for materials. We’ll post updates here after the system has been running for a while when we can gauge how well it performs.

August 1, 2010
We have beenrunning the cooling system continuously for six days now, and it seems to have reached equilibrium because the temperature in the cistern is holding steady at 66.7 degrees F, which is 3 degrees warmer than when we started. The whole cistern is not at that temperature, just the water in the vicinity of the heat exchanger. As we transfer heat from the house into the cistern water, it warms up a little and transfers the heat down into the earth. When the cistern water’s temperature is no longer changing, then it’s transferring heat into the earth at the same rate we’re removing it from the house, and we should be able to sustain that rate indefinitely.

We switched the pumps down to their lowest speed setting a few days ago, in order to reduce their electricity consumption. That reduced the total cooling capacity of the system a little but it also improved the energy efficiency. So here are the numbers: water coming from the floors measures 73 degrees, and water coming from the cistern measures 68 degrees so its temperature drops 5 degrees as the water flows through the copper tube heat exchanger in the bottom of the cistern. The pumps are moving 1.8 gallons of water per minute on their low setting (where they each consume only 60 watts), which means the system is removing about 4100 BTU/hr (1200 watts) of heat from the house. That gives us an EER of 34, about 3 times as efficient as a good air conditioner and nearly 2 times as efficient as a good heat pump system! More importantly, the house is staying comfortable at 75 degrees or less despite outdoor temps in the upper 80s. We’re using 2.9 kW h of electricity per day to run the circulating pumps, which is significant when our typical usage is around 10 kW h per day, but we would only expect to run this system about 60 days per year so the cooling load represents only about 2.5% of the annual energy production from our solar electric system. To put it another way, our cooling system will consume as much electrical energy in an entire season as a large central air conditioner system consumes in one day!

June 15, 2012
Continued: Enhanced Cooling System