11 June 2006

Thermal Storage, an Efficient Allocation of Resources?

Does the Lovins/passivehaus building construction theme render the concept of thermal storage systems for regulating the intermittency of renewable energy sources obsolete? In order to examine this question we would need to first compare the capital investment of each system. However, since neither is deployed in an quantity, this isn't really possible. The issue that can be analyzed then is the ancillary value of thermal storage to a renewable electricity grid versus the efficiency gains of the passivehaus concept.

Passive Home Concept

For those of you who have never come across the Amory Lovins schtik or the German Passivehaus building standards, it has been shown that residential or commercial structures can be built with practically no heating requirements. This is done through construction that is properly insulated and sealed with minimal air exchange and uses passive heating (or cooling, depending on climate) strategies. The passivehaus benefits from entropy in terms of home heating. Every electronic device is essentially a resistance heater in addition to its functional purpose, and every person an 80-100 W thermal source.

Scale of Thermal Storage

The most likely medium for thermal storage is water due to its low cost, heat capacity, and the fact that it is liquid and hence is easy to transfer heat with it.

The specific heat capacity of water is 4.184 KJ kg-1 K-1. The heat of fusion − the energy required to change the phase of water from solid ice to liquid water − is 334 KJ kg-1 or the equivalent of almost 80 K. Relative to 20 °C, ice stores a greater amount of cooling power than boiling water.

Consider data on space heating and air conditioning for the USA in 2001. I'll use the worst case: Northeast homes for heating and Southwest for cooling. I am going to ignore hot water heating even though it's significant because it's not relevant to the argument in the end. The average Northeast home uses 63 mmbtu/year for space heating. This works out to an average of about 0.18 GJ/day or 0.365 GJ/day during the peak heating season assuming some sinusoidal distribution. This is the equivalent of 87,000 kg K/day of water; if we store the water at 80 °C to heat the home at 20 °C then we need approximately 1.5 tonnes of water, or 1.5 cubic meters worth (nearly 400 gallons).

On the cooling front, the average Southwest annual electricity consumption is 4,000 kWh/year. If we use an average coefficient of performance of 3.o then the actual cooling supplied is 0.12 GJ/day or a estimated peak of 0.235 GJ/day. If, again, we assume the house is kept at 20 °C then 2/3 of a tonne of ice is required. Consider that 1-2.5 tons (of ice) are common ratings for a centralized air conditioning systems.

Taken over North America (say 100 million homes), these seem like significant numbers. 44 mmbtu of natural gas at $10/mmbtu is $44 billion dollars a year and the production of ~67 Megatonnes of CO2 . 2,300 kWh of electricity for cooling is a total of $18.4 billion dollars (at $0.08/kWh) per year and assuming coal power (at 900 g/kWh), 207 Megatonnes of CO2.

The North American GDP is about $12 trillion per year, so residential heating and cooling alone constitute 0.5 % of that. Scale-wise, there is plenty of potential for passivehaus or thermal storage systems. But can they be friends?

Heat Pump Efficiency

The coefficient of performance is how much heat is moved for a given amount of work (electricity in this case). Commercially air conditioners in the USA are rated based on a Energy Efficiency Rating (EER) which is the square of the COP between 80 F and 95 F (or about 300 K and 308 K). There is also a Seasonal EER (SEER) which is a different (more relaxed) standard. The theoretical COP for this temperature range is 37, but in reality most systems are in the range of around 3.5. The ultimate theoretical coefficient of performance of a heat pump is given by:

COP = TH / (TH - TC) = TH / ΔT

Herein lies a problem. As the temperature difference a heat pump has to cross increases its efficiency decreases. Normally an air conditioner only has to work across 8 K or so. However, if we want to use it to make ice, from a night-time temperature of 24 °C, then the ultimate efficiency of the system will only be a third normal. For a real-world system the drop in efficiency on a percentage basis will not be so precipitous but it will still be disadvantaged trying to make ice.

Overall it's a tough sell for thermal storage as a means of handling renewables intermittency. As we've seen, thermal storage sets efficiency against grid regulation. Generally, when we have schemes with competing criteria they fail to be economically attractive. Witness my investigation into solar thermal cooling. In that case there was a competition between the efficiency of the solar thermal collector and absorption chiller on the basis of temperature. Here we have competition on pure power. Yes, we can store off-peak power, but the effective round-trip efficiency is going to be unimpressive simply due to the drop of in performance of the heat pump.

There is still the possibility to run numerous appliances on a deferrable basis. Heat, air-conditioning and refrigeration all only need to maintain a given (if narrow) temperature range so with good insulation they should be able to run on relatively short duty cycles. Other appliances, such as the dishwasher or combination washer/dryer can be scheduled.

Chicken or Egg?

One problem with pumping efficient homes is that houses last for such a long time. One often heard meme in the peak oil world is that car fleets take too long to be replaced. Houses can be renovated, cars can't. Still if you are one of those people who think suburbia is evil (as opposed to just soulless) then the choice of whether to concentrate on improving the efficiency of houses or cars presents quite a quandary. From my point of view, I'm more concerned overall with climate change and more localized pollution of the air and water. If people want to live in rows of identical pink stucco houses... enjoy.


Randy Semrau said...

I suppose that if Anthropogenic Global Warming were actually a real issue, and if it were, that we could persuade the new megagiant global polluters to get on board the AGW Bandwagon, there might actually be some point in trying to 'Do your Part'. In reality, you should be doing this only because it makes sense to your own pocketbook.

Brian in Phoenix said...

In regards to your argument of efficiency in heat pumps; freezing water at 32F is very costly indeed! However, freezing water at 44F is not nearly as costly due to the narrower Delta T, at least not with the addition of sodium bromide or other added materials to raise the freezing point of water at atmospheric pressures. Solar (or heat pump) Heat retention (storage) is also made more practical with certain additives (phase change materials or PCM). Intensive research is being conducted to be the first guy (or girl) on the block to come up with an economical and ecologically sound version of both, or combination substrate. All of this is great! Except........if you are going to do anything at all, start with insulation! A well insulated dwelling will reduce thermal infiltration (exfiltration), requiring smaller amount of energy to be consumed in the first place. It also sets the stage for a less costly change over of systems due to the lower energy requirements; therefore good insulation has the added effect of reducing future equipment costs.
In addition, I'd like to comment on the air conditioning requirements for the American southwest; I live in Phoenix and yes, I'm an air conditioning contractor. The Peak Heat load average for Phoenix works something like this (and this is a very rough rule of thumb): 1 ton of air conditioning is required for every 450 sq. ft. of living space to keep the indoor ambient at about 75F during peak summer months. That figure drops to about 1 ton for every 250 sq. ft. for commercial buildings due to occupancy and ceiling height. This means that the average home of 1800 sq. ft. requires a 4 ton unit. Taking into account that we had 38 days at or above 110F this past summer, we have enormous daytime peak energy use. My local utility charges me 17.8 cents per KWH for that peak time and 4.7 cents for off peak time. Peak time is from 1pm to 8pm Monday thru Friday, May 1st Thru October 31st. This energy (Cooling and heating) alone amounts to about 1MWH/year in my home as it is an average sized dwelling. If on peak usage is 20 percent of week and off peak is 80 percent, then I could save an estimated $26.20 per summer month just by shifting my A/C run time to all off peak. This does not include calcs for heating which would increase my annual saving, especially if I could utilize the same storage vault to store solar heat for night time (on peak) winter heating. I estimate that a $300.00 minimum saving would be realized as well as having improved the overall equity in my home (something people tend to forget when considering alternative energy plans. Assumption: a $3000.00 (projected cost of tank and storage medium) investment would not only pay for it's self over 10 years but would increase my homes value and reduce over-all energy usage (remember that the heat pump will be running at a lower delta T)for the given amount of BTU's exchanged. What are your thoughts?

Robert McLeod said...

Interesting to hear that your utility has a split rate in Arizona. Obviously, that's a necessity to makes the concept for thermal storage possible.

I would try to estimate maintenance costs for your system as well. Generally I assume that the cost of energy will increase at the same rate as inflation, so investments in energy infrastructure can often make good sense. As long as you are stable in your business and expect to stay in the area it's probably a good deal.

Obviously with the greater tendency of people to refinance these days investments in their home may be less likely to pay themselves back. People probably will pay a premium for a fully certified 'green' dwelling but you're looking at a smaller section of the buyers market in that case.

I'd be curious to know if your utility also allows net metering? At $0.178/kWh you should be able to payback a PV system relatively quickly in Phoenix (~ 15 years, depending on the inverter mostly). RETScreen says you'll get about 1.8 kWh/year per peak watt of rated power in Phoenix for 12 % efficient cells.

Cyril R said...

Robert, the recent commercialization of CO2 transcritical cycle air heat pumps may be an answer to the tradeoff. Contrary to normal heat pumps, they're actually more efficient with bigger delta T's. Bizarre, but they're already being sold by the hundreds of thousands in Japan. So efficiency and grid regulation can go hand in hand.