Heat Pumps and Net Zero Energy homes


My friend Jack asked me recently about geothermal heat pumps. I have been looking into heat pump technology for a while and wanted to write about it, so I gave him a longer-than-usual reply and figured I’d put some of what I wrote in my blog for anyone else who was curious about ground source heat pumps and net zero homes in general.

Heat pumps are like air conditioners running in reverse. You can use a heat pump to either heat or cool a building by reversing the flow of its refrigerant. Just as air conditioners become less efficient when the outside temperature gets too high, heat pumps get less efficient when the outside temperature gets too low (like around 30F). Of course, this is the worst time for a heating system to start losing its efficiency, that is, as the outdoor temperature gets colder, because it’s precisely when the maximum output from it is required. This is one of the reasons heat pumps have not been as popular in colder climates as they are in mild climates.

The benefit of adding a geothermal source to a heat pump is that the heat exchange loop stays at a consistent temperature. This allows the heat pump to maintain its efficiency because the earth below the frost line at most latitudes in the lower 48 states stays at a consistent 50-60 degrees F year round. This constant source of temperature allows heat pumps to maintain a high ‘coefficient of performance’ (COP). The COP is similar in concept to an air conditioner’s SEER (Seasonal Energy Efficiency Ratio). Basically, the COP is a ratio of watts of electricity input to watts of heat output you get. A typical heat pump has a COP of around 3 if the difference between the indoor and outdoor temperatures is within about 40 degrees F. The less the temperature difference, the higher the COP and conversely, the larger the temperature difference, the lower the COP. A COP of 3 is like getting 300% efficiency compared with simple electric resistive heat which is 100% efficient. A 100% efficient heater has a COP of 1. However, when the outdoor temperature approaches freezing, a heat pump’s COP can drop down below 1, at which time a resistive backup heater takes over. The primary drawback of an air source heat pump is that just when you need heat the most, a heat pump starts to get much more expensive to operate due to the reduction in its COP. For geothermal (i.e., ground source) systems, the COP is closer to 3.5 all the time and so it doesn’t suffer from the problem with air source heat pumps that vary with outdoor temperature.

An air conditioner’s efficiency is measured by its SEER which is basically the COP averaged over a range of typical outdoor temperatures and multiplied by 3.413. Typical values for an air conditioner’s SEER are around 10-15 which corresponds to an COP range of 2.9 – 4.4 . It’s been improving over the past few years, mostly due to government mandates. In Japan, they are now producing heat pumps with COPs as high as 6.

To add a geothermal heat sink to a heat pump system, you need to bury the heat exchanger loop below the frost line. This can be done using a loop in a vertical bore hole or in a horizontal trench. In general, a ground source heat exchange loop for a typical house would be between 1500 to 2500 feet long depending on the size of the system, and buried at least 6′ deep. The costs to install the heat exchange loop are similar to those of drilling a well in the case of a vertical system, or digging a 6′ deep trench several hundred feet long and two feet wide. So the installation of the heat exchange loop can get quite expensive. If the loop is arranged in a coil in a trench, you need about 1 foot of trench length per every 4 feet of loop. As you can imagine, this would not be easy to do unless it’s done during the construction of the house and prior to any landscaping. Also, if anything goes wrong with the loop such as having a leak, it would be very expensive to isolate and fix the problem once it’s buried.

If heat pumps are 300% efficient, why doesn’t everyone use them? After all, a gas furnace is only abut 80% efficient. One reason is that generating electricity from coal, gas, or nuclear power is only about 30-40% efficient. As much as 2/3 of the thermal energy created at an electric power plant becomes wasted heat. So the overall savings due to the multiplicative effect of the heat pump are offset by the losses of converting the fuel to electricity back at the power plant, not to mention the losses of delivering energy over the electrical grid. That’s part of the reason that electricity tends to cost about 3 times as much per unit of energy as buying natural gas and burning it in a furnace to heat your house. The capital and installation costs of a geothermal heat pump system are also significantly more than a gas furnace (about $25K vs. $3K).

I currently spend about $800 per year to heat my home with natural gas and a similar amount on electricity. I figure if I were to use electricity and a geothermal heat pump for heat, it would cost about the same as what I currently pay for natural gas, but I’d have an extra $22K in capital cost for the heat pump over the cost of a gas furnace. Now, if I had a total solar electric home then it would make sense to consider a heat pump, but to do that, I’d need to have about 12 kW of solar panels installed on my roof (at a cost of $86K), based on my annual gas and electric consumption. Even with generous solar rebates (currently $4.50/W by my utility company, up to $45K) and the new solar tax credit just passed by the U.S. Congress (up to 30% of the net solar system cost) that could take my cost of the solar system down to around $29K. But it still would be hard to justify because of the additional capital outlay for the heat pump, bringing the system cost up to $54K.

To get to net zero energy with my existing home using PV solar and a ground source heat pump, it would take about $111K in capital expenditures of which $57K could be foisted off on to my fellow taxpayers and utility customers. But that’s still too expensive to justify it based on its economic return. It would take about $54K in personal expenditures to save $1600/year in utility bills. Ignoring the cost of financing a $54K expenditure, the amortization of the system would be $2700 per year assuming it needed to be replaced in 20 years. But if energy prices doubled, which is certainly possible, it would begin to look much more attractive. They’d need to quadruple for it to be attractive without subsidies.

Energy efficiency initiatives don’t get a lot of attention because most of them have negative economic returns. This is usually due to the low cost of energy in the U.S. which is about half of what Europeans pay due to higher energy taxes. However, if you use energy efficiency as a way to reduce capital costs of a renewable energy system, the picture is quite different, primarily because renewable energy capital expenses are so high. My electricity and heating bills are currently on par with the U.S. average. If I could figure out how to reduce them by 50%, it would allow me to reduce the size of a renewable energy system proportionally to be half the cost. This is usually when efficiency becomes a much more attractive proposition. Getting by with a 6 KW solar system for all our electric and heating needs would cut the previously mentioned $86K pre-rebate cost in half.

If energy prices go up significantly, and there’s good reason to believe they will as oil and gas production peak, you’ll likely see a lot more uptake in the technologies that help create net zero energy homes.

National Renewable Energy Lab visit


A few weeks ago I visited the National Renewable Energy Lab open house in Golden, CO with a few other members of the Northern Colorado Clean Energy Network. I’d wanted to see this facility for some time, and figured that an open house on a Saturday would allow some of our members who normally are unable to attend our energy tours during the week to join us. As it turned out, we only had 4 members of our group show up. Despite the low turnout, we had a good time carpooling there and back because we got to chat for a few hours about renewable energy topics.

The NREL has a visitor’s center and there was a presentation in progress when we arrived about how to do an energy audit on one’s home. Several of us had just been to an NCRES presentation on this topic recently so we did not sit down to listen to the presentation. The presentation took up much of the visitor’s center display area, making it impossible to talk without disrupting the presentation so our ability to wander around inside was a bit limited.

The exhibits were very nicely constructed and a docent explained the various renewable energy programs underway and the purpose of the various buildings on the campus. There are numerous projects going on all over the facility, but unfortunately they are off-limits for visitors. Only the visitor’s center is accessible. I had expected this to be the case, and so I tried to gather some information about what would be necessary to get a tour of the actual laboratories in the hope that some future visit would allow us to get better access to what’s going on in the labs. I can see that it will be a challenge as they are not set up to handle tours of the actual labs.

The docent who was our guide had spent most of his career in the power field, and I had a long discussion with him about transmission of power over high voltage DC lines. Transmitting power over DC lines is counter-intuitive for most engineers who were taught that you can only transmit utility scale power on AC lines. But thanks to advances in high power semiconductor components to handle utility scale power, DC power transmission lines are becoming more common to deliver electrical power long distances and to help isolate grids through interties. This method of transmitting power will become more important in the future as some of the best potential sources of renewable power generation such as wind and solar tend to be far removed from population centers. HVDC power transmission has the advantage of being able to isolate the grids so that the need to control the phase of the AC power over long distances is not required. The largest DC line in the U.S. is the Pacific DC Intertie which takes hydroelectric power from the Columbia River in Washington State and delivers it to customers in the Los Angeles area.

My favorite Visitor’s Center exhibit was the section of the GE 37-meter wind turbine blade. I’ve seen these blades up close during our Ponnequin Wind Farm tour, but was curious about the materials of construction. With the section exposed, I saw that the materials looked identical to those used in my LongEZ and Cozy. They consisted of wood, foam, fiberglass, and epoxy albeit on a much larger scale that what is used in my planes.

Me and Ed Miccio standing next to the GE blade section

You can see that the spar and caps are very thick on these blades.

The Cozy uses the same materials and construction techniques as the wind turbine blades.

The NREL visitor’s center is open from 9-5 Monday through Friday and I’d highly recommend that if you ever find yourself in the vicinity of I-70 at exit 263, you should stop by for a short visit and self-guided tour.

Colorado Water Resources


A few months ago I did some research and wrote some blog postings about hydroelectricity in Colorado. I had been asked by my friend, Bevan, whether we were failing to take advantage of the hydroelectric power that was available from the rivers in Colorado simply because of the political issues associated with damming our beautiful river canyons. In doing this research, I found that we do, in fact, harvest some of the hydro power and, due to the fact that the flow rates of these rivers are not large or consistent, we would not really gain much power generating capacity even if we extracted all of their theoretical hydroelectric energy.

One of the most fascinating public projects I read about during my hydroelectric research is the Colorado Big Thompson water diversion project. Using a series of tunnels, pipes, canals, reservoirs and pumping stations, this project collects and diverts water from west of the continental divide and brings it to the eastern slope. About 70% of the population of Colorado lives along the Front Range, yet 70% of the precipitation falls on the western side of the continental divide. The C-BT project provides about 213,000 acre feet of water to the eastern slope each year. Nearly all of this water has its energy extracted through a series of electric generating stations with a combined capacity of 162 MW. That’s enough electricity for about 80,000 homes. It also provides enough water for about 425,000 homes. To put it in perspective, the C-BT project delivers more water to the Front Range than both the Big Thompson and Cache la Poudre rivers combined.

An acre-foot of water is about 326,000 gallons. Each household in Colorado uses about .5 acre feet per year which about 13,600 gallons per month. This is about 30% more than the national average, which is due to the need to irrigate our lawns. Colorado has a very dry climate and in order to have a lawn and shrubbery, they must be irrigated. It made me wonder how much water we use for things that are essential compared with uses those that are not essential, such as growing lawns.

A general rule of thumb is that each person in the U.S. uses about 50 gallons of water per day. You can estimate your daily consumption by visiting a USGS site and using their calculator. The calculator uses the following values for personal water consumption:

  • Bath: 50 gallons
  • Shower: 2 gallons per minute
  • Teeth brushing: 1 gallon
  • Hands/face washing: 1 gallon
  • Face/leg shaving: 1 gallon
  • Dishwasher: 20 gallons/load
  • Dishwashing by hand: 5 gallons/load
  • Clothes washing (machine): 10 gallons/load
  • Toilet flush: 3 gallons
  • Glasses of water: 8 oz. per glass (1/16th of a gallon)

Another way measure your household’s water consumption is to look at one of your water bills from a winter month. I found that our water consumption comes out very close to the estimate of 50 gallons/person per day. The real shocker for me was looking at a summer water bill and comparing it to a winter water bill. Our summer water consumption goes up by a factor of 10! For about 4 months out of the year we need to run the sprinkler system and its water consumption dwarfs the amount of water for personal use during those months. Overall, watering our lawn for those 4 month accounts for more than 65% of our annual water consumption!

I began to wonder what this is costing us so I began to study our water bills. Interpreting utility bills is not always easy. There are sometimes so many charges that it’s hard to tell what drives the overall cost. I had to call our city’s water department to figure out how the charges are computed. In the case of our water bill, there are three charges. The first is for the storm sewer, which is based on the size of the property. The second is for the regular sewer bill, which is determined by water consumption during a winter month to eliminate the effect of irrigation water, which doesn’t return to the sewer. The last is the cost of the water used based on a meter reading to measure actual water consumption. Included in the water charge is a flat connection charge, which is around $8/month. When you combine the two sewer charges of $18 with this $8 charge my water bill is already at $26/month before I’ve purchased my first gallon.

The cost per 1000 gallons of water in Greeley is $2.41, which is about the average in U.S.. That’s up about 40% from what we were paying 6 years ago, so it’s been increasing faster than inflation. For those of you in other countries who measure water in cu. meters, there are about 264 gallons per cu. meter.

I visited the manufacturer’s web site for my sprinkler system and found out that each 360-degree sprinkler nozzle uses about 3 gallons per minute. The quarter and half nozzles use proportionally less water per minute. I have 9 sprinkling zones each with a total of about 5 “360-degree equivalent” heads, so when I’m watering my lawn, I’m using about 15 GPM. My watering cycle takes 3 hours so that comes out to 2700 gallons. At the $2.41/1000 gallon cost, it costs about $6.50 each time the sprinkler cycles. We’re restricted to 3 days a week that we can water the lawn, so that adds about $80/month for watering the lawn in the summer time. Now that I know how much each watering costs, I’m being more vigilant about using the timer’s ‘rain’ button to suspend watering when we’ve just gotten some rain. I’ve even been looking at the weather forecast to see if it makes sense to skip a cycle if rain is predicted.

Sometimes people have asked if we can do something more intelligent when it comes to watering lawns, such as using ‘gray water’, i.e., the water that would normally be sent to the sewer and directing it to water the lawn instead. That might work for water that is lightly contaminated such as water from a shower or dishwasher, but there is no easy way to separate that from the other contaminated water that you (and your neighbors) wouldn’t want on your lawn. We also need to consider that waste water from inside the house is eventually treated and put back in rivers where it can be used downstream. Also, now that I know that it takes 10 times as much water to keep the lawn green as the amount we need for personal use, I can see that recycling gray water would hardly put a dent in one’s overall water consumption.

How about collecting rain water from the roof and other surfaces and storing it? In my case, only about a third of our 1/2 acre lot has grass on it. The rest is covered with impervious surfaces like the house, concrete patios, the driveway, and landscaping rock. If it were possible to capture the rain water, would this work to offset or even eliminate a watering bill? I did the calculations and there does appear to be enough precipitation that falls on this lot (about .5 acre-foot per year) to supply all of our watering needs. However, to store and treat this water would not be practical. A single lawn watering takes 2700 gallons which comes out to 8000 gallons per week. Since it can sometimes go for weeks without any significant rain during the summer, we’d likely need a 20,000 gallon storage tank to store $50 worth of water. Then you have to consider that it would take chemicals to keep it from turning into a bacteria pond and it’s easy to see why cisterns have never proved to be very popular when tap water is available. There are even laws about capturing one’s own rain water in Colorado since water rights and property are separate and so it is against the law to capture and hold your own property’s rain water. Here’s an article about water harvesting in Colorado that contains more information about it.

The other option is
xeriscaping which means having a lawn with plants that can survive with no supplemental irrigation water. However, this is not always possible and the attractiveness of this approach will no doubt vary with the eye of the beholder. My friend Peter lives in a subdivision where the covenants require the residents to have a certain percentage of green grass in their lawns. Some people say that they love the look of natural desert, but to be honest, it’s only beautiful at a distance. The natural ground cover on Colorado’s Front Range is mostly noxious weeds full of pointy things that will pierce your skin. There is not much attractive about what grows on Colorado’s Front Range naturally. Most people think of Colorado as beautiful mountains filled with Aspen and pine trees. That all starts about 30 miles to the west. Most of us live on the plains.

The availability of water is starting to limit growth in this area and if we get a serious drought, it will likely cause a further restrictions on new growth. The new water tap connection fees are already in excess of $14,000 per home in Greeley.

People like living in dry climates because it’s almost always sunny and there’s very little humidity. But we all need water to survive and to create an attractive environment. We all like having green grass and shade trees nearby. We have plenty of land in Colorado for future growth, but not enough water to support unrestricted growth. Every gallon of water I conserve will likely get used up by some new construction project that is enabled by the water’s newfound availability. It’s quite a dilemma about what to do when it comes to water conservation. Everyone wants to do their part, but if the reward for it is more growth and more people, then that takes some of the incentive out of it. We could grow the population of Colorado until we’re all walking around in stillsuits, but what good would that be?

Having said that, I do realize that certain industries like construction depend on new growth to survive. I hate to be like the people who, once they have found a promised land, put up a no trespassing sign and tell everyone else to stay out. That’s not an uncommon sentiment to hear people express in this area. The city of Boulder has had an anti-growth policy for many years. Everyone wants to be the last one in.

Colorado is somewhat unique among the dry western states because we have areas in the state that get in excess of 50 inches of precipitation per year and areas that get less than 10 inches per year. Most of the areas where people live get between 10 to 15 inches per year, which is not enough to grow much more than cactus, thistles, and tumble weeds. To put it in perspective, states east of the Mississippi get between 40 to 50 inches of precipitation per year and it’s quite consistent throughout the region. When you get over about 40 inches per year, it’s usually not necessary to irrigate one’s lawn. In Colorado, most of the high precipitation areas are the mountain peaks, which tend to hold the precipitation throughout the winter in the form of snow and release it gradually during the spring runoff. This runoff is captured in a number of reservoirs and used during the dry summer months for residential, commercial, and agricultural use. It’s a very delicate balance that requires carefully matching the supply with the demand.

The problem with precipitation is that it is local and seasonal. In other words, it’s difficult to match the amount of precipitation you get with where you need it, when you need it. And that problem is compounded in states like Colorado where the population and seasonal effects of precipitation are not matched very well. We need to be very resourceful about how we collect, distribute, and use the water resources we have. And one must not underestimate the beneficial environmental impact of paving corn fields and constructing strip malls in their place, an activity that has continued unabated in Colorado over the past decade.

That leads me to my last observation. Is agriculture on a high desert plain an intelligent use of land and water? I’m sure that for people who are involved in farming that they’d consider it to be the most beneficial use of the land. They’ll no doubt maintain that attitude until someone offers them several hundred years’ of annual farming profits for the property to construct a residential neighborhood or a strip mall on the land. In the case of high density living where one builds apartments, this would definitely qualify as a net water savings. Irrigated crops in this region take about 1.3 acre-feet of irrigation water per acre on the average, whereas if you put about 12 people on that acre, it would take less than half of the amount of water, especially if you pack them in so that you don’t have much lawn to water. If you pave the parking lot and streets around the neighborhood, all the better, because the water that falls on it can be collected and used elsewhere. Similarly, virtually all the water that crops use evaporates, but most of the water people use gets treated and put back in the river just a few miles away, so it can be used downstream. I do realize that water that evaporates will eventually get recycled, but unlike a river, it’s a lot harder to maintain claim to it once it goes into the sky.

So it would appear that for every acre of agriculture we give up, we can jam another 12 residents into Colorado. Then all we need to do is find some jobs for them.

Landfill Gas-to-Energy Tour


On Thursday, May 15th several members of the Northern Colorado Clean Energy Network toured the Denver Arapahoe Disposal Site to see a new landfill gas-to-energy project currently under construction and nearing completion. The site is constructed on a concrete pad that had previously been used for remediation of materials removed from the Lowry landfill, a superfund site, which is adjacent to the DADS landfill. For a period of around 13 years, beginning in 1967, the Lowry landfill received more than 100 million gallons of liquid chemical waste. I had incorrectly assumed that this Lowry superfund site was in some way associated military activity because it shares its name with the former Lowry Air Force base, but it was actually created by legal dumping at the city-owned landfill. The landfill’s name is likely a result of its proximity to the Lowry AFB Titan 1 Missile Complex 1A which is just to the west of it.

In addition to touring the facility, our tour guide also drove us up on to the top of the landfill where large earth movers were organizing the waste into ‘cells’ that were compacted and covered up with dirt. The dirt helps to keep the trash from blowing away and reduces its odor. The mountain that they are now constructing in this section of the landfill will eventually reach a height of 300 feet. If you want to see an aerial view of the site, here’s a link to Google Maps. You can see the Lowry landfill in the lower southwest section, a completed portion of the landfill that is 150′ tall in the northwest section, the new part under construction in the north east section, and a decommissioned Titan 1 nuclear missile site in the southeast section. If you zoom into active part of the site with the Google Maps view, please note the number of earth movers you can see on the site. That helps to give you a perspective on how big this site is.

The methane gas that will power the 4 16-cylinder 1100 HP Caterpillar engines is piped from various sections of the landfill. This gas is currently being flared (burned) and relased to the atmosphere in compliance with government regulations. Once the plant has been commissioned, the gas will be re-routed to the engines where it will be used to generate electricity. The current flow is 1200 cfm and that can produce 3.2MW of electricity which is enough to power about 3200 homes.

Landfills leak methane gas as the organic materials buried in them decompose and so if it’s not collected, it’s possible for it to accumulate and if it does that, it can become an explosion hazard. Even if the methane were not to accumulate, it would eventually find its way into the atmosphere and methane is about 25 times more potent as a greenhouse gas than CO2 so it’s better for the environment to collect and burn it and turn it into CO2.

The gas that comes out of a landfill is actually about half methane and half CO2 with small amounts of water along with minute amounts of other gases. The water needs to be removed from the gas to prevent it from corroding the engines and there is an apparatus that uses alternating heating and cooling of the gas to condense out the water. The water removed from the gas is sent to a water treatment facility. The diesel engines have been specially modified to run on a mixture of methane and CO2.

This apparatus removes the water from the landfill gas

One part of the facility that I found particularly interesting were the controls that took the electricity and converted it for use on the grid. They used Woodward controllers and large cabinets with impressively large bus bars and capacitors. It was one of the parts of the facility where no pictures were allowed.

The 4 engines are currently 16 cylinder models but the facility is sized so it can be re-fitted with 20 cylinder engines that would produce twice as much power should the gas flow continue to increase. Because of Denver’s arid climate, the gas flow from these sizable landfills is just a fraction of what it would be in a moist climate. This is a disadvantage in some ways, but a benefit in other ways. It takes a much larger landfill in an arid climate to make economic sense for electricity generation, but it should also produce methane for a longer period of time, because it will take longer for the organic material inside the landfill to decompose. At the current rate of gas production, the existing wells should produce for another 20 years. There is enough land available for many decades before this landfill would be considered ‘full’ and the newer mountains will be much larger than the existing ones and so this landfill may be producing electricity for many decades.

Several of the 1100 HP Caterpillar engine/generators

This landfill is owned by the city of Denver and receives about 1200 truckloads of solid waste per day. It operates 6 days a week, 24 hours a day. In addition to burying trash, there is a concrete recycling operation on site where old concrete is ground up and used over again, saving cost on materials and energy over making concrete from scratch. There are several other recycling operations on the site.

I came away from this trip impressed with the engineering that has gone into designing and maintaining a modern landfill. We have come a very long way from just a few short decades ago when we though it was environmentally responsible to handle liquid chemical waste by simply dumping it in an out-of-the-way place, not realizing that a city would eventually grow out to meet it. To be fair, at that time the links between the long term health effects of exposure to toxic substances were not well known and so many waste handling policies of that era were formed out of ignorance. This site is continually monitored to make sure that nothing hazardous makes its way into the water table or atmosphere.

We’d like to thank Brad Gagne and Steve Derus for making this trip possible and for answering the numerous questions we had about the facility. Everyone felt like they got a lot of out seeing an operation like this up close.

Some members of NoCoClean and our host, Brad Gagne, in the engine room.