written by:
November 16, 2011

Project Manager Brad Liljequist chronicles the building of the zHome, a ten-unit townhome in Issaquah, Washington—the first multifamily zero-energy community in the United States. Part 5: How do ground source heat pumps and solar panels work?

 

Two of our most central technologies in achieving zero net energy are our ground source heat pump system (for heating and hot water), and our solar panels (which generate electricity). The two account for about 60% of getting to zero net energy, so obviously they play a key role. 

Ground source heat pumps are a well-known technology, but are generally not mainstream, especially here in the Pacific Northwest. The system combines three highly efficient processes which together result in a system which over three times more efficient than a typical forced air furnace. The slides give a good narrative to how the system works, but if you’d like more details, check out the ground source system sign from the zHome education signage—it is the second sign in sign package one.

Solar energy, surprisingly, works quite well in the Northwest—solar panels here put out about 70% of the solar energy of a panel in Sacramento. Solar energy quietly is becoming more and more cost effective, with prices coming down and efficiency going up. Currently solar panels convert about 15-20% of the solar energy hitting them to energy—quite efficient when you consider that photosynthesis is only a half a percent efficient! Also, solar panels are quite durable—many panels from the 1970’s are still functioning well.  There is little to go wrong in them. Given how little maintenance they require (simple occasional  cleaning) there is a huge amount going for them.
 

The first stage of the system is the ground source piping system. At zHome, we have fifteen 220-foot-deep boreholes, where we’ve inserted U-shaped one inch diameter pipes grouted into place and fused into one interconnected set of pipes, just below the gr
The first stage of the system is the ground source piping system. At zHome, we have fifteen 220-foot-deep boreholes, where we’ve inserted U-shaped one inch diameter pipes grouted into place and fused into one interconnected set of pipes, just below the ground surface. Through these we pump a water/ethanol mixture, which absorbs the average ground temperature of 51 degrees. This fluid is then pumped into each unit, where it is then further heated by the heat pump. The benefit of doing this is that the heating system is effectively always heating from that 51 degrees—as if it is in a perennially moderate climate.
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Here, we are inserting the piping into one of the six inch diameter bore holes. We had very difficult ground conditions on our site and the drilling took ten weeks. The piping is high density polyethylene—the same type of pipe used for natural gas piping—
Here, we are inserting the piping into one of the six inch diameter bore holes. We had very difficult ground conditions on our site and the drilling took ten weeks. The piping is high density polyethylene—the same type of pipe used for natural gas piping—and thus extremely heavy duty.
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The ground source wells are tied together via a set of manifolds fused together a few feet below the ground surface. Between additional solar electrical conduit, the ground source piping, rainwater piping, and normal utilities (stormwater, water, electric
The ground source wells are tied together via a set of manifolds fused together a few feet below the ground surface. Between additional solar electrical conduit, the ground source piping, rainwater piping, and normal utilities (stormwater, water, electrical, and sewer) we have an amazing amount of pipe in the ground at zHome.
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This is grand central for both heating and hot water. The heat pump is the silver/blue box on the right, into which 51 degree wellfield fluid is fed (you can just see the black wellfield piping to the right of the heat pump). The heat pump does its work,
This is grand central for both heating and hot water. The heat pump is the silver/blue box on the right, into which 51 degree wellfield fluid is fed (you can just see the black wellfield piping to the right of the heat pump). The heat pump does its work, and 125 degree hot water is circulated out of the heat pump and into the white tank. Water from this tank is then used for hot water uses as well as in floor heating. Each zHome has its own heat pump and tank.
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Heat pumps are often explained in overly complicated ways, but actually operate off of a very simple chemical law: when something in a gaseous state is compressed, it heats up, and when it decompresses, it gets cold. Have you noticed how a bike pump, or a
Heat pumps are often explained in overly complicated ways, but actually operate off of a very simple chemical law: when something in a gaseous state is compressed, it heats up, and when it decompresses, it gets cold. Have you noticed how a bike pump, or a backpacking stove tank gets hot when you pump it up? Or similarly, how an aerosol can gets cold when you spray it? Those are examples of this law in action. A heat pump simply mechanizes this. Incidentally, every home has a heat pump—it’s what makes your refrigerator cold.
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Heating is distributed hydronically in the floors. A major benefit to hydronic heating is that it is much more efficient to convey heat energy via a fluid than via a gas. To the left is a mockup of our ground floor (finished concrete on top of insulation
Heating is distributed hydronically in the floors. A major benefit to hydronic heating is that it is much more efficient to convey heat energy via a fluid than via a gas. To the left is a mockup of our ground floor (finished concrete on top of insulation on top of the ground) and to the right is the upper floor system (FSC-certified bamboo over a quick track system). Ichijo makes the quick track system themselves, which is used in all their homes. The hydronic distribution also factors into our integrated design approach of no wall to wall carpet—an efficient heat distribution system goes hand in hand with eliminating a place for dust and toxins to collect.
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The fluid in the ground source wellfield is circulated via two highly efficient, German made, one horsepower Grundfos Magna pumps. These have been classified by the EU certification system as grade "A" in terms of efficiency. These variable speed pumps ra
The fluid in the ground source wellfield is circulated via two highly efficient, German made, one horsepower Grundfos Magna pumps. These have been classified by the EU certification system as grade "A" in terms of efficiency. These variable speed pumps range in power output all the way from 35 to 900 watts, meaning they are able to fine tune their speed based on demand from the community.
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All told, zHome’s solar panels will put out about 70,000 kwh hours annually. At peak production, the whole array will generate nearly 70,000 watts. We used panels manufactured by Solarworld in Hillsboro, Oregon. Solarworld is a well established German com
All told, zHome’s solar panels will put out about 70,000 kwh hours annually. At peak production, the whole array will generate nearly 70,000 watts. We used panels manufactured by Solarworld in Hillsboro, Oregon. Solarworld is a well established German company with factories around the world. We were impressed with their build quality, ability to support their warranty, and wanted to support with regional economy with a high value added product. The Hillsboro plant is a full tilt, vertically integrated factory where they even grow their own silicon.
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We used Enphase microinverters to transform the direct current produced by the panels into alternating current that can be exchanged with the electrical grid. Arrays which are on single larger inverters are impacted by shade on individual panels which red
We used Enphase microinverters to transform the direct current produced by the panels into alternating current that can be exchanged with the electrical grid. Arrays which are on single larger inverters are impacted by shade on individual panels which reduces the overall array performance. Microinverters separate each panel from the other so these shading impacts are minimized.
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Another benefit to microinverters is user feedback—the Enphase system comes with an online service which allows you to monitor solar production of each individual panel in real time and historically.
Another benefit to microinverters is user feedback—the Enphase system comes with an online service which allows you to monitor solar production of each individual panel in real time and historically.
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This is a bank of meters for three of the units plus the community system (community loads such as the ground source wellfield are also offset for zero net energy). The meters are the right are production meters. These count every watt of solar energy pro
This is a bank of meters for three of the units plus the community system (community loads such as the ground source wellfield are also offset for zero net energy). The meters are the right are production meters. These count every watt of solar energy produced by each home’s array. The State of Washington pays fifteen cents a kilowatt hour for solar energy production. Each zHomeowner will receive a $800-$1000 check annually for their solar production. On the left are the net meters. These are like normal electric meters, but these spin forward AND backward, depending on whether the zHome is producing or consuming electricity on a net basis.
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Here’s the net meter readout for the Stewardship Center. Even with lights on and computers plugged in, on a cloudy day it is net producing energy, shown by the arrow pointing to the left. At this moment 699 watts are being sent back to the grid.<br /><br
Here’s the net meter readout for the Stewardship Center. Even with lights on and computers plugged in, on a cloudy day it is net producing energy, shown by the arrow pointing to the left. At this moment 699 watts are being sent back to the grid.

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The first stage of the system is the ground source piping system. At zHome, we have fifteen 220-foot-deep boreholes, where we’ve inserted U-shaped one inch diameter pipes grouted into place and fused into one interconnected set of pipes, just below the gr
The first stage of the system is the ground source piping system. At zHome, we have fifteen 220-foot-deep boreholes, where we’ve inserted U-shaped one inch diameter pipes grouted into place and fused into one interconnected set of pipes, just below the ground surface. Through these we pump a water/ethanol mixture, which absorbs the average ground temperature of 51 degrees. This fluid is then pumped into each unit, where it is then further heated by the heat pump. The benefit of doing this is that the heating system is effectively always heating from that 51 degrees—as if it is in a perennially moderate climate.

Click here to read past installemts of Building A Zero-Energy Community.

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