Low/Zero Energy/Zero Carbon

Due to a combination of factors there is much interest in reducing the amount of energy, and in particular fossil fuel energy ("carbon emissions") used in the industrialized countries.  The major forces driving this is the increasing concern due to climate change potential caused by burning fossil fuels, the fact that the majority of the worlds petroleum is located in unstable countries and the environmental concerns associated with drilling, mining, transporting and burning fossil fuels.

There has been an increased awareness of how much energy is used in buildings, and as a result, there are a number of proposed standards for dramatic energy reduction:

  • The 2030challange (architecture2030.org) calls for staged elimination of carbon emissions by 2030.
  • The living building challenge (ilbi.org) calls for buildings to be energy self-sufficient using on-site energy
  • The passive house institute (www.passivehouse.us) is a very low energy standard used widely in Europe (particularly the colder, but not very cold areas).
  • Zero energy building is a movement to make buildings that use zero energy over the course of the year (but typically using the grid as seasonal energy storage).
  • Zero carbon buildings is a more strict version of zero energy: no carbon can be emitted.  In this version any offsite energy generation must be non-fossil fuel, and the on-site generation must compensate for transmission losses that come with using off-site power.

Unlike other topics on this site which are more of the "if you do this, you get this" variety, the material on this page is somewhat more controversial, not only in which approach makes the most sense, but in how much needs to be done at all.   While the public has generally embraced energy conservation, dramatic reductions as proposed here are not mainstream, and in fact some huge segment of the population doesn't believe global warming exists.1

Aside: A brief note on "global warming" and fossil fuel use

While my intent for this document is to be as objective as possible (avoiding political or value judgments), I don't find most discussions on global warming to be particularly objective.  There are so many problems with using fossil fuels at the rate we're using them (strip mining, acid rain, smog, oil spills, development pressure on wild lands for gas and oil, and unfriendly oil-rich nations to name the most obvious), that whether not global warming exists seems like a moot point.  The real argument about global warming is really about whether it actually exists, but whether we have to do anything about it, with the deniers having a high concern about economic collapse, and those on the environmental side being more concerned with environmental collapse (while also seeing economic benefits in restructuring infrastructure).

While my bias is far toward the environmental side of the equation, and probably most people who are reading this are similar, this is aimed at everyone else.  Its an attempt at a rational discussion that the majority of people will find reasonable.

First, its important to understand that virtually every aspect of lives and much of our economy is currently run on fossil fuels.  The issue then is: can we cut that fossil fuel use to near zero and not suffer from a greatly reduced standard of living.  There are two aspects of this (1) how much can we convert fossil fuel use to renewable energy use (2) how much energy use can we live without.  The first question is mostly one of technology, willpower, and willing finance.  The second question is more complex, because it calls into question the relationship between wealth and happiness.  This topic is addressed by Bill Mckibben.2, who notes that although our wealth has increased since 1950, our happiness has not gone up.  Likewise, we in the United States do not appear to be anywhere near the happiest people on the planet.  Of course, this is very subjective, and many will undoubtedly argue the opposite.

No one argues whether there is more carbon dioxide in the atmosphere, in fact we have very solid data that its at its highest point in at least 100,000 years. The arguments is whether or not its relevant.  Carbon dioxide is a very small component of the atmosphere, it's not the only greenhouse gas, and many other factors affect global temperature3

The issue here is the earth's heat energy balance, which behaves like a passive solar house, where the atmosphere performs a similar function to windows.  Energy from the sun comes in as a mixture of short-wave IR, visible light and UV; it is absorbed by the atmosphere, water vapor, land and water; it is then re-radiated as long wave IR.  Except for a narrow band, most long wave IR is absorbed by water vapor, CO2, methane etc ("green house gases"), and it then re-radiated again in all directions, the net effect being that some of the escaping heat is reflected back (and some of the heat absorbed by water vapor will return when it falls as rain).  The net effect is to increase the surface temperature of the earth (and of course the air also).  However, as the earth's surface temperature increases, it radiates more energy to the sky, balancing the reflection of the green house gases; the net effect is that the surface temperature raises until a balance point is hit where the outgoing energy equals the incoming energy.  The one monkey wrench in all this is that some of the sun's incoming energy is reflected by clouds, so the balance point is also moved by the average area of clouds.

So while is obvious that doing all these calculations is non-trivial, it should also be obvious that if you change a parameter (for example the amount of green house gases in the atmosphere), that the temperature set point will also change unless something else changes to counter the first change.  So the questions are (1) how much does the set point change in relation to the change in greenhouse gases (2) as the set point changes, does the cloud cover change enough to significantly reduce how much the temperature set point goes up?

Before addressing these questions, its important to understand the difference between weather, climate and global climate.  Weather, although dependent on climate, is mostly determined by what large circulating air masses pass by a given location.  Climate is the averages of those weather events over a long period of time.  Global climate is then the average of all the local climates over the entire surface of the earth.  The main difference between climate and global climate is that global climate must obey a strict energy balance: the average global temperature is dependent on that energy balance.  Climate for a location is dependent only on the air masses that pass by it, hence it is possible for the global temperature set-point to increase, while a given climate can get colder.  Obviously for this to happen, other climates would have to be proportionally warmer to account for a warmer global climate.

Modeling climate is done with the same general method as modeling weather, but here we're only interested in long term averages, not specific weather.  We divide the atmosphere into parcels somewhere between 1 and 10km on a side, and then divide that into various layers, including a base layer of either ocean or land.  Being more accurate requires smaller cells and more detail, so approximations are used.  To compensate for that, models are run with various different approximations, in case they are wrong.  Then each model is run out many years to see what happens.  If most models say that a particular outcome will happen, then we have some confidence that its true, although we're never 100% sure.  Needless to say, the details are fairly complex, so to some degree its comes down to whether you trust climate scientists or not.

Another way to look at it is to do a thought experiment imagining the planet's energy flow.  Starting where we're at, add green house gases.  Lets even assume that the added gases only increase the insulation value by 1%.  So what happens?  Since the increase in insulation is small, in the short term, probably not much, so then the question is, does the planet get warmer over time?  In order for the answer to be "no", there has to be some mechanism that exactly counteracts the increased insulation.  If you think of it in terms of risk factor, the question then becomes, what is the risk that the extra insulation won't be exactly balanced?  Then ask yourself if you'd take that risk if your life depended on it.

The two way of looking at increased green house gases also going by the terms climate change and global warming.  Technically global warming is the most accurate term for what we think will happen with increased green house gases, but we also suspect that the warming will not be uniform.  Some places might even get colder, even though the average over the entire planet will be warmer.  Likewise rainfall patterns are likely to move along with weather patterns.

For those who think that a little warming is good thing, or that extra CO2 will just make plants grow faster are being a bit optimistic. Plants tend to be mildly fussy about soil type, moisture content, and hours of sunlight.   Some plants like apples, and stone fruit need a certain period of cold to set fruit.  They are generally somewhat tuned to the conditions they evolved in.

What are we risking?  Species that can migrate will, but those that can't will suffer--how much depends on how much the climate changes and how sensitive the species are.  Crop failures will be increasing more likely, even with technology.  Farmers will change what they plant and farms will migrate with climate over time, but it won't happen over night--for example, its not straightforward for a Kansas wheat farmer to move to Saskatchewan--its a different country.  The biggest issue for humanity is our sheer number and our high reliance on technology to feed the world--we have not given ourselves much breathing room.

With a problem this large people tend to feel there is little they can do, but this is not the case.  If each person did a small thing, multiplied by six billion people (or depending on your viewpoint, maybe just by the one billion who have more opportunity to do something) then multiplied by a twenty year period, the impact would be huge.  While ignoring the problem is an option, it carries it's own risk factors.

My final thought is that anyone looking at the situation strictly objectively would be hard pressed not to suggest that reducing world population is likely to be as effective as any other solution, although there are so many practical, political, cultural and ethical problems in actually accomplishing this that it isn't likely.  One can only hope most of the six billion of us will individually decide this is a good idea.  In the meantime western societies can retool their economies for increased energy efficiency and use of renewable energy, giving us some time to reconsider whether our lifestyle is really making us happy or not, and whether there is a reasonable tradeoff between out current energy intensive lifestyle and the simple, but laborious lifestyles of our ancestors. 

Zero Energy/Carbon Neutral

There is much talk about "zero energy" homes and "carbon neutral" activities.   A zero energy house generates all the energy it needs over the course of the year, while a carbon neutral building generates no carbon.  The difference is subtle and a bit nitpicking since few buildings even come close to zero energy and has to do with the use of grid electric as a backup source.

Since electric is the only readily available renewable fuel, these houses are generally all electric.  The issue with grid-electric isn't that it isn't technically carbon neutral, but it does rely on the existence of a grid, and that grid is currently no where near carbon neutral.  The two options for the purist is to either buy renewable electric from the grid, or not to use the grid at all.  The latter solution is, for example, required by the living building challenge.

It would be equally valid to use a biomass based fuel that itself was generated in a carbon neutral way (for example using methane from animal waste, or sustainably harvested wood), but this requires finding such sources and since there is a very limited market for them, it will likely be a long time before sustainable biomass is available as a home energy source.5

The implicit assumption behind zero energy is essentially that generating energy on site makes more sense than community generation--this is never stated, but why else would zero be the goal, as opposed to just very low?  The truth is probably somewhere in between, and the best strategy is likely a combination of local generation and renewable energy purchased from the grid.  While the power companies seem to favor very large scale generation in remote locations (typically 500mw or more), many smaller facilities may make more sense, if for no other reason that building giant transmission lines is costly, unpopular and often not environmentally preferable.  In addition, smaller local generation facilities allow the possibly of using the waste heat to supply space heat in the neighborhood.

Practical Details: all electric homes have some limitations due to the low energy content of electric compared with natural gas.  The main one is that electric hot water heaters are about four times slower than gas ones, and for an electric tankless hot water unit to be equivalent to a gas one, it requires its own 100amp circuit, which will require an oversized service panel.  Heat pump hot water is more efficient (2-3 times more), but has its own set of complications (in particular that it delivers heat very slowly).  The only other issue is that many people prefer to cook on gas stoves.  Electric induction ranges may make most of them happy, or alternatively a market for carbon-neutral natural gas (methane) could develop.  If the later happened, it would still be possible to use gas burning fireplace inserts and stand-alone stoves, although I suspect the cost of carbon neutral gas to be much more than the current cost of fossil fuel gas.

Anyone who has experience with off-grid living knows that even with passive solar, solar hot water and PV, some propane is usually used for cooking and/or hot water.  For the grid-connected, the issue is whether the grid can be migrated over time to 100% renewable power. While many question this possibility, there is increasing evidence that it is possibly using a nationwide grid of wind, solar, hyrdo, geothermal and possibly some kind of storage.  The key is load matching: for example, southern areas can generate more solar than they need in the winter, and northern areas can ship their excess solar in the summer back south; excess wind in one area can be shipped to somewhere the wind isn't blowing.  Combined with dramatic energy efficiency in each building and some local generation, the US could at least come very close to using 100% renewable electric.4

Aside: Issues with a Fully Renewable Grid

While on the small scale one can argue that it is possible to buy back renewable electric, for example by being part of a green power program, the reality is that this solution doesn't scale up beyond a small percentage of users, and in fact the only reason you can buy back reliable power is because most of it comes from easily controllable fossil fuel plants (for an in depth look at electric generation and carbon emissions, see: http://tonto.eia.doe.gov/FTPROOT/environment/co2emiss00.pdf).  Currently, the grid has no storage.  This isn't to say green power programs aren't a good idea--they are--it's just all they do is put more renewable electric on the grid...they don't solve the big picture problems of building a renewable grid.

In fact, it is the grid will have to undergo a significant restructuring to handle large amounts of distributed power. Here are the issues:

  1. The storage issue must be addressed. Unless there is always enough renewable generation at all times to cover the load, storage will be needed.  Current technology batteries are expensive and have relatively low energy density, although if widely deployed, they may be sufficient to cover daily fluctuations.   One solution along these lines that has been consider is using the batteries of hybrid cars as storage by converting them to "plug in hybrids" (I admit to not having done enough research to comment on this approach).  Another frequently discussed approach is using hydrogen, which to be carbon-neutral would have to come from hydrolysis of water or from a biomass source, which could then be burned in a fuel cell to make electric.  The major downside to this is that neither conversion is currently very efficient, although the fuel cell process can be efficient if there is a use for the waste heat it generates (for example to heat water, or the house itself).  In the meantime, renewable energy generation is so small that we can generate large quantities of it to the grid before much has to change.

  2. The grid is not designed for distributed generation.  Currently the grid is essentially a mesh of tree-like structures: large amount of power are moved on big lines to substations and from there is it distributed on trunk lines, which are then divided into branch lines.  A branch line only has to carry the average amount of power needed by the customers it serves, a number that is typically much smaller than the maximum each can use because at any given moment most people are using only a very small amount of their maximum.  When this assumption stops being true, like during a summer heat wave when everyone turns on air conditioners, the grid occasionally fails.  If every building were to generate solar electric, the reverse situation can occur (for example on a sunny summer day in Seattle when everyone is at the beach!) causing the grid to carry more than its maximum amount of power--only backwards!  There are, of course, multiple solutions: store it locally, convert it to hydrogen, install bigger wires, etc.

  3. If we're going to move large quantities of electric between regions seasonally, we will undoubtedly need many more high power transmission lines.  Most (all?) northern communities are unable to generate enough electric for their daily winter use.  Since no one likes these things, some effort will have to into installing this capacity in an environmental and community sensitive way.

  4. The grid has losses--about 7-10% of the generated power is lost, so for a home to be zero-energy, it must generate enough extra to cover these losses.  Since the grid currently moves power one way, I would assume that moving it two ways (eg once on generation, and then a second time to buy it back), would involve twice the loss.

This is not to say the problem isn't solvable, just that the solution will require rethinking how our power grid works in addition to adding voluminous amounts of renewable capacity (including conservation!).  For a more in-depth view (and slightly different as well), the American Solar Energy Society published a big picture solution to getting to carbon neutral at  http://www.ases.org/climatechange/climate_change.pdf

My short version would be: 

  1. We need to look at every possible renewable energy technology.  Unfortunately even renewable energy has environmental impact, so we must be careful to understand what that is before deploying them on the grand scale.

  2. At regular intervals, review each renewable technology, deploying and researching the ones that make the most sense at the time. The reason is that at this point, no single renewable technology appears to be able to solve the entire problem.  Since the technology behind them is not mature, one can expect many improvements over the next 20-50 years.  Funding needs to be based on performance potential, not short term economic potential. Our current policy makes no sense: we throw money at whatever the current panacea of the moment appears to be (say hydrogen or ethanol), but its all motivated by short term profit alone, not potential or clear thinking.  By providing random incentives, we allow the current short term thinking of the US financial markets to dictate our national energy policy.  It becomes up to the private sector whether to fund research, manufacture products or purchase them. Instead we must be looking for the best economic policy that makes an efficient transition to renewables with the least amount of pain.

  3. Finding out what the best scale is to deploy the renewable generation.  We know that distributing the generation is likely to lead to improved reliability, but we also know that homeowners are notoriously bad at maintenance.  The cost of retrofitting the grid for distributed generation must also be considered.  If power plant scale turns out to be too big, then we need to know if millions of distributed generation points is too small.

The ASES study is the most logical approach I've seen so far, but is not an outline that many environmentalists will find completely satisfying because it only addresses the issue of atmospheric carbon.  At issue is the other environmental costs of renewable energy, in particular what is the environmental cost of deploying renewable energy on undeveloped land that is still a functioning ecosystem supporting biodiversity and providing respite from the onslaught of development.  We've already dammed most rivers, so now we look toward covering thousands of square miles of land with wind farms and thousands more with concentrating solar power plants.  These technologies certainly solve the carbon problem, but unless their use is limited to already developed land, those of us who value natural spaces will find these solutions a very bitter pill to swallow.

Biofuels are a promising approach, in particular those based on agriculture or wood waste, but like every other solution, an investigation into the negative environmental effects of the massive use of biofuels needs to be done, especially given that our entire agricultural system is currently heavily petroleum based.  Any massive change in the way biomass moves thru the ecosystems could result in serious consequences.

Finally, no discussion of climate change can ignore the issue of nuclear power, since it is effectively a zero carbon alternative.  In my view, nuclear power has never even approached living up to its promise of "energy too cheap to meter".  Not only are the power plants expensive and complex, but the issue of nuclear waste has found no decent solution after thirty years or so of looking for one.  The fact that the "waste" is actually so physically hot (not to mention radioactively hot) that it must be stored in water to keep it cool, indicates that these plants are not at all efficient.  From the waste perspective, the only reasonable solution is a breeder that actually uses all its fuel...although I have never seen such a reactor described: the breeder still produces high level waste--just less of it, and with generally shorter lifetimes. Unfortunately the breeder (as well as all reactors) use the exact same fuel (U235 & PU239) that is used in bombs, the possibility of nuclear proliferation makes breeders only more dangerous than the standard reactor.

Given the choice of nuclear power, covering thousands of square miles of land with wind farms, and/or much of southern Arizona with concentrating solar plants, most environmentalists will probably pick "use less power", but of course that may not be one of the choices offered.  Difficult choices like this will probably be the key battles over the next twenty years.  One can only hope that thru a combination of technology improvements, proper economic stimulus, and social change that we will be able to find a solution that is acceptable to the majority of people.

There are many other schemes to achieve carbon neutrality, including planting trees, burying carbon etc.  While they may or may not be able to get carbon out of the atmosphere, these solutions don't address the fundamental issue: that by using fossil fuels, we're consuming very old solar energy that is not replaceable in any reasonable time frame.  To achieve true carbon neutrality we will have to stop using fossil fuels virtually completely. (note: the only scheme for burying carbon that could make sense would be if the carbon comes from biomass. This is essential using plants to remove carbon and then storing it somehow.  Personally I'm pretty skeptical sequestering will work, unless there is some way to lock it up chemically (like as calcium-carbonate (limestone)), and even then you have to think about the natural geologic processes that might chemically unbind the carbon.)

Zero Carbon Theoretical Case Study

The following is an theoretical analysis of how to build a zero carbon home. The essential principles are simple:

  • use as little energy as possible
  • generate what you can from solar7
  • use other renewable sources/storage to provide the rest.

In this analysis, the idea isn't to create a single zero carbon home, but how to convert all of them, meaning that grid electric can only be relied on if it can be converted to 100% renewable sources.  It is the need to convert the grid (or avoid using it) that makes converting every home to zero carbon difficult, especially when cost effectiveness is considered.

If the assumption is that grid electric will someday be 100% renewable, then being zero carbon is just a matter of installing enough PV, so that over the course of the year, the building uses zero energy.  This is the strategy taken by most zero-energy homes to date. The problem with that strategy is that if we don't minimize what we use and generate as much as possible on site, we might be facing hundreds of square miles of now wild and rural land covered in wind and solar generation facilities: better than a global warming world, but as we are finding, not without their own set of environmental impacts.6  Even if all the energy is generated on site, it is still desirable to make every building as efficient as possible to allow for excess generation on some sites to compensate for other sites where there is too much shading.

The other alternative, to be off-grid is also very challenging, and may not even make sense on a large scale: for example, it may be far more cost effective to generate renewable energy on a neighborhood scale than it is to restrict generation to the building site.8  In general the off-grid experience shows that its relatively easy to store heat and electric for the evening, and just a little harder to get thru a short cloudy spell, but is very difficult to get thru anything longer than that.  Most current off-grid houses use some combination of propane and wood as backup energy--both not possible in a zero-carbon, urban setting.

Mild sunny climates are easiest, followed by cold sunny climates and mild marine climates, while any climate with limited sunshine is going to be very hard.

On the upside, a home's energy use can be lowered dramatically (probably by 80%) from current average (2010) levels, by a combination of efficiency and some local generation.

House Design

Heating Climates:
The first step is to determine how much passive (and active solar) can be captured, then see if it is practical to build an envelope that will hold keep the house at a comfortable temperature using solar alone.  This is non-trivial in a large part of the US, so the more practical solution is to build the most insulating envelope practical.  Once that point is reached, the question is whether it is cheaper to put additional improvements into the envelope, or invest in energy generation.

Calculate the anticipated electrical load on a yearly basis and size the PV system to match that yearly demand.  Now that two-stage heat pumps are available that will function at least semi-efficiently down to near 0F, this is the most efficient way to provide heat.  An alternative that also works in colder climates is to use a ground-source heat pump.

Finally, look for ways to reduce demand when the sun isn't shining, or store energy.

Cooling Climates:
Start by reducing the building's heat gain as much a practical.  Unlike heating climates, there is generally solar energy during at least some of the cooling demand, so the PV can be sized to cover both the normal electrical load and the air-conditioning load.  It is also possible to store "coolth" in the form of ice (at least one product on the market), which uses electric to make ice during one part of the day, and then melts the ice for cooling during another (they are actually being sold as devices to lower peak load: they run at night, and melt ice during the day).

What do houses use now?  A fairly efficient moderate size house built recently in a moderate climate would use on order of: 40 million BTUs (11800kwh) of heat energy, another 4 million (1200kwh) in hot water, and 5500kwh of electric (19 million BTUs).  The total is 63mBTU (18,500kwh).   A less efficient, older house might use twice that, and in a colder climate maybe even four times that.

The off-grid lesson.  People living in off-grid homes have learned to get by with a lot less.  They not only use super efficient lighting, but also super-efficient appliances, they fanatically avoid phantom loads, and even more fanatically turn devices off when not in use.  The reason is simple: PV energy is very expensive.

Quite a number of wall designs have been tried that produce an approximate R40 wall at affordable costs, and R60 ceilings are just a matter of leaving enough space (for example, see Fine Homebuilding, Jan 2010).  An air tightness of 2ACH50 is generally achievable, and with a  more experience it could be brought down closer to 1ACH50.  Dual pane U.3 windows are now more available, and although U.2 windows (and even much better) are available from a small handful of manufactures.

Doing all this will bring energy use quite low, and then solar will hopefully supplement the rest.

Backup Heat: For short term heat use in sunny climates, thermal mass should be sufficient, but for longer term, the only zero-carbon source of backup heat that makes sense for large scale use is the electric heat pump.  Energy storage beyond the typical battery back and thermal mass is currently not practical.  The general problem with energy storage is density,  it just takes up too much space to store the amount of energy you need during long periods of limited sunshine.  Seasonal storage using rock bins has been tried a number of times, but requires an enormous amount of rock (on order of 50,000 cubic feet or more).  Using water is also difficult, although a very wet, very porous material (say 50% water) kept at only a moderate temperature and tapped with a heat pump might be vaguely possible, but would still be very large.In either case, the best way to get heat out of storage is a heat pump, so then why not just use a ground source heat pump?  The obvious answer is that storing heat reduces the size of the field, but then you have to insulate the storage area, and the hotter you try to make it (some get over 140F), the faster heat will leak out.


1:  my cynical side thinks is the same huge segment that failed science, but in reality its probably more that they don't want to believe and scientists have done a bad job in convincing them.

2: Deep Economy, Bill Mckibben, 2007

3: the part that seems hard for people to understand is that the planet is like a thermos bottle with a heat source.  Even a 1% increased heat retention will result in raise in the temperature inside over a long enough period of time.  With climate change, we're talking about 2-4°F over 10 to 20 years, which is a very small change indeed.

Further, there is the belief that factors like increased clouds may serve to reduce global warming.  Other than the issue of questioning a model that most of us don't understand, the general problem I have with this line of thinking is that it seems like wishful thinking to assume that the interacting factors will produce a benign result rather than a bad one.

There is also the argument that the models aren't accurate yet, so we don't have to do anything. While its true that the models are far from perfect, not doing anything is clearly taking some risk, and as previously stated, it really comes down to whether you feel you have a lot to lose in taking action.

It is also possible that we are seeing a natural cycle, although there is no real evidence for it (on the contrary, the evidence seems to indicate we're due for a cooling cycle).  If this is true, the negative outcome is the same, just that it doesn't matter if we keep burning fossil fuels.

4: admittedly this is a dramatic change in both building and grid operations, will probably involve some changes in occupant behavior, and will be a non-trivial change.

5: the one exception is biofuel based heating oil, but there is so much controversy about whether this is even vaguely sustainable, I'm ignoring such sources.

6: this isn't to say that current zero-energy projects haven't minimized energy use already, since the expense of PV pushes the designer in that direction.  Its more that we have think a lot harder about where our grid electric is going to come from. The battle over sites for wind and solar has already begun.  Transmission lines are being fought also.

7: or wind etc if you have them available, but since I'm assuming an urban location, and wind turbines need to be 30' above the nearest object, wind towers in urban locations are not currently acceptable in most zoning regulations.

8: this is one of my main complaints about the living building challenge.

9: imagine using 4' under the basement/slab for storage (so for a 1000SF footprint, its 4000 cubic feet).  If its 50% water, that's 2000ft3 water, or 124,000lbs, assuming the other 2000ft3 is stone of some sort (ie 140lb/ft3,.2Btu/lb), so that's equivalent to another 56,000lbs of water.  Kept at only 20F above ground temp, that stores 3,600,000Btus, which is a lot, but in even with an incredibly insulated house you probably need to store twice that, and maybe 4 times that.