Sunday, February 22, 2015

2015 heating cost comparisons

A couple years ago I did some calculations to compare heating costs in Nova Scotia.  Since then energy costs have changed a bit, and I have more accurate information on the efficiency of oil boilers and pellet stoves.  As well I intend to add natural gas to the comparisons.

Electric heating costs have not changed much, with the cost of electricity now 14.95c/kWh.  This equates to a cost of 4.38c per thousand BTUs.

Furnace oil is now selling for 95c/L.  In my previous calculations, I assumed a 90% efficient condensing boiler.  These are uncommon in NS, so I'll use the 84% efficiency of an oil fired boiler with a tankless coil.  This equates to a cost of 3.14c/kBTU.  Pie anyone?

Instead of propane which is not commonly used for space heating in NS, I'll look at the cost of natural gas.  The current price of natural gas is $20.69/GJ.  When the $21.87 monthly charges is factored over my estimate of 42GJ/yr of gas consumption for a moderately energy-efficient residence, the total cost per GJ is $26.93/GJ.  With one gigajoule equal to 948 kBTU, and an efficiency equivalent to an oil fired boiler, natural gas heat costs 3.38c/kBTU.

For wood pellets, prices have increased so that 40lb bags are selling for at least $5.50.  I've also found out that wood pellet stove efficiency tops out at around 87%, and for typical units is closer to 75%.  After updating my calculations based on the higher price and lower efficiency, wood pellet heat costs 2.29c/kBTU.

With the recent popularity of air-source heat pumps in Nova Scotia, it is prudent to compare their cost of heat to other sources.  A high-efficiency unit with a COP of 2.4 will provide heat at a cost even lower than pellets - 1.83c/kBTU.  A lower efficiency unit with a COP of 1.4 will provide heat for about the same cost as oil - 3.13c/kBTU.

Tuesday, December 30, 2014

Solar PV economics - approaching grid parity in Nova Scotia

In recent years, the cost of solar PV panels has dropped from over $3/watt to under $1/watt.  The drop in prices has even caused China's LDK Solar to declare bankruptcy.  Surging exports of low-cost Chinese-made PV products has lead to a tariff war that will likely put a halt to big price drops in the US and EU.  Assuming Canada does not follow suit and impose high tariffs on Chinese imports, we should see panel prices of C$0.75/W by the end of 2015.  Even at current prices, I'll explain how PV is getting close to grid parity in places with relatively high electricity costs (15c/kWh).

Most of the solar power industry in Canada is focused on Ontario, due to the high subsidies under the microFit program.  At 39c/kWh, a rooftop PV system is a no-brainer.  The cost of the panels and an inverter to convert the DC power into AC adds up to about $1.50/kWh for a 8kW system.  Installation costs can vary depending on how high and steep the roof is, however I think around $5000 for a 8kW system is a reasonable price.  If a solar installation contractor wants to charge much more than that, I'd consider hiring a roofing contractor to mount the panels and an electrician to install the wiring and inverter.

Although the cost of solar panels has dropped by about 75% in the last five years, there has not been an equivalent reduction in the costs of inverters.  Given the costs of the input materials - the solar wafers, glass, metal frames - I think PV panel costs will bottom out around 50c/W.  With inverters, the technology still has room for significant improvements.  Google's Little Box challenge is one example of incentives to improve inverter technology.  Within the next five years, I expect the cost of grid-tie inverters to drop from over 50c/W to under 20c/W.  This along with more competition on the PV installation market should bring the total installed cost including taxes of a residential PV system to under $1.50/W, compared to around $2.50/W now.

So at current prices, a 8kW system would have a total installed cost of about $20,000.   How long that cost is amortized over has a big impact on the economics.  Solar panel warranties are usually 25 years.  Their efficiency drops over time as well; after 25 years about 80% of the installed efficiency is common.  Warranties on inverters are much less - 5 or 10 years.  For financing, the longest amortization for mortgages available in Canada now is 25 years.  Therefore, I think a 25-year amortization makes the most sense.

Interest on a 10yr fixed mortgage with a 25 year amortization is about 4.4%, and the monthly payments on that mortgage would be $109/month.  The PV pontential of most of Eastern Canada is around 1000kWh/kW.  That means a 8kW system would generate about 8000kWh of electricity per year.  With a cost of electricity of 15c/kWh, that would generate an average of  $100 worth of electricity per month, almost covering the $109/mth costs of the system.

One caveat for Nova Scotia is that the current grid-tie tariff does not allow you to produce more electricity than you use.  An energy-efficient house, unless it uses electric heat, would likely use less than 8000kWh of electricity per year.  Smaller systems have less economies of scale, so a 5kW system would likely have a cost of $3/W.  Grid parity may not be here yet in Eastern Canada, but it is coming soon.

Sunday, September 28, 2014

Mini split heat pumps

My regional electric utility has been promoting air-source heat pumps, and  my friend Dan who works installing these types of units tells me they have become quite popular over the last few years.  I discussed the efficiency of geothermal heat pumps in my heating costs comparison post a few years ago, so I figured it's time I did a similar analysis of air-source heat pumps.

As with any type of heat pump, the bigger the temperature difference (called lift), the lower the efficiency of the heat pump.  The efficiency rating for air-source heat pumps is usually given as a heating seasonal performance factors (HSPF).  This is a seasonal average of BTUs of heat provided per watt of energy consumed.  To convert HSPF to COP that is the usual performance rating for geothermal heat pumps, divide the HSPF by 3.4 - the number of BTUs per Watt.

HSPF by itself is not a useful performance measure, since it depends on the heating season outside temperature.  If the unit does not specify the temperature for the HSPF, it is likely 8.3C (47F for those who don't think in metric).  This might be a useful measure for someone living in Vancouver, BC, but not so much for someone living in Halifax, NS where the average January temperature is about -5C.

NrCan states:
At 10°C, the coefficient of performance (COP) of air-source heat pumps is typically about 3.3. This means that 3.3 kilowatt hours (kWh) of heat are transferred for every kWh of electricity supplied to the heat pump. At –8.3°C, the COP is typically 2.3.

A COP of 3.3 equals a HSPF of 11.2, and a COP of 2.3 equals a HSPF of 7.8.  So if your heat pump has a HSPF rating of >7 at -8.3C (17F), it will produce heat for less than half the cost of an electric resistance heater.  The hard part is finding out that efficiency rating.

Mitsubishi Mr. Slim is a popular mini split system, so I tried to find the full specifications for it's efficiency.  I couldn't find them on Mitsubishi's web site, but I was able to find them on a Mitsubishi reseller's web site.  Page 13 has a chart with the efficiency of several of the heat pump models, but the HSPF is only given for 17F.  There are performance numbers given at other temperatures, and since HSPF is BTUs per Watt times 3.4, the HSPF can be calculated at different temperatures.

I started with the GE24NA, a nominal 2 ton unit with a HSPF of 10 at 8.3C.  At -8.3C, it outputs 16,000BTU and consumes 3290W, for a HSPF of only 4.9.  This equates to a COP of 1.4, far short of the typical 2.3 COP stated by NrCan.  Compare that to the D30NA, a nominal 2.5 ton unit with a lower HSPF of 8.2 at 8.3C.  At -8.3C, it outputs 19,500BTU and consumes 2400W, for a HSPF of 8.1 (2.4 COP).  For heating a home in Nova Scotia, the D30NA is a much more efficient unit.

Another reason the D30NA is a much better choice is because at 19,500BTU it will be able to provide more of your heating needs than the GE24NA at 16,000.  Both units will probably need supplemental heat on the coldest winter days.  If you heat your house with electricity, you can look at your electric bill to figure out your heat load, remembering that 1 Watt is 3.4 BTUs.  If you can't find your bill details, but remember your electricity costs, you can figure it out from that.  For example a $450 electricity bill in January when electricity costs 15c/kWh means your consumption was 3000kWh, or 10.2 million BTUs of energy.  Dividing by the number of hours in the month gives an average of 14,000 BTUs per hour.  During a January winter storm with high winds and temperatures of -20C, you'll likely need more heat than the GE24NA can put out.

To analyze the economics, it helps to look back at my heating costs comparison post.  Electricity and oil are slightly more expensive than they were two years ago, but not by much; heating oil is selling for $1.07 per litre.  Wood pellets can still be found for $5/bag.  That puts the cost of a BTU of heat from electricity at about 1.4 times oil and 2.5 times wood pellets.  The conclusion is that a decent air-source heat pump is cheaper than heating with oil, and about as cheap as heating with wood pellets.  If you can get the time-of-day tariff and use a smart thermostat to avoid using electricity during peak time, the heat pump will be even cheaper than wood pellets.

Monday, June 24, 2013

Nuclear furnace for home heating

Since I was a teenager I've been interested in cheap energy sources.  In the past couple years, I've read a few articles and seen a couple Ted talks on thorium reactors.  I thought it would be simpler, cheaper, and more efficient to use the heat from a nuclear reactor directly in the home instead of converting it to electricity first.  As a result, I'm planning to test it out.

I had read about a kid who used lantern mantles as a source of thorium, but that seemed like too much work.  Then I read about thoriated tungsten welding rods.  A local welding supply shop sells a 10-pack of 2.4mm x 175mm rods for under $40.  Each rod has a volume of 0.8cm^2, and .3g of Th.  Based on the articles I've read on thorium reactors, the heat generated from the nuclear reaction of 1g of Th is 36 million BTUs.  So if I can consume all the thorium from 10 rods (3g), I'd generate 108 million BTUs of heat, at a cost of under 40 cents per million BTUs.
In my post on costs of heating in NS, I calculated that heat from electricity costs a little over $40 per million BTUs.  So the cost of heating with a thorium nuclear furnace should be about 100x cheaper than electricity!  Besides thorium, the other thing I need to make a nuclear furnace is a neutron source.  The liquid salt thorium reactor articles talk about using uranium-233, which I can't (legally) obtain.  Ka-Ngo Leung and his colleagues in Berkeley Labs have invented a cheap way to generate neutrons, but it's not commercially available yet.  The radioactive boy scout stories say he used radioactive americium-241 from smoke detectors as his neutron source.  He wrapped it in aluminum, which absorbs the alpha particles from the americium and spits out neutrons.  I'll try the same thing.

I also need a neutron moderator to slow down the neutrons so they'll be captured by the thorium atoms to start the nuclear reaction.  Hydrogen, carbon, and to some extent oxygen all make good moderators.  Candu reactors use heavy water, but that's hard to get and it's expensive.  Most nuclear ractors use regular water.  The radioactive boy scout used charcoal (carbon).  Paraffin wax is mostly carbon and hydrogen atoms, and so makes a good moderator.  I'd like to be able to easily remove the neutron source (to turn off the furnace).  Paraffin wax would melt when the furnace heats up, so my first attempt will be to wrap the neutron source with some charcoal using some aluminum foil.  I'll attach a wire, and drop the cylindrical neutron generator into a tube that is surrounded by the thoriated tungsten rods.

I'd love to hear from any physics heads on what the rate of the reaction should be.  Protactinium-233, the decay product of Th-233 has a half-life of 27 days and beta-decays into U-233.  So I'd guess it will take a couple weeks to approach full temperature.

Saturday, March 16, 2013

Heat Pump Hacking

My primary heat source is a 3-ton water-to-air geothermal heat pump.  It was factory-filled with R-22 (freon).  It was designed for warmer climates, as it had a factory-installed freeze protection switch that shut the unit off when the outgoing water temperature came close to 0C.  I bypassed the freeze switch and used a water and antifreeze (windshield washer fluid) mix for the loop.

The optimal amount of refrigerant in heat pumps (and air conditioners) depends on the temperatures of the cold and hot sides. I wanted to tweak the refrigerant charge, but R-22 is bad for the atmosphere and hard to come by.  I came across some information on propane (r-290) as a refrigerant which indicated it can be used as a substitute for R-22.  I can get it cheap at my local hardware store, and even Greenpeace likes it.

I read about people using propane for DIY computer cooling, and someone that recharged an R-22 system with propane.  The first thing I needed was a manifold gauge set.  They tend to sell for $100-150, but I found what I thought was a good deal on ebay for about $50.  It has plastic handles on the valves, and one was broken on arrival.  About a minute after I hooked it up to the high and low side schrader valves I heard a loud pop.  It took several more minutes to figure out one of the hoses had burst and was leaking.  Now I would HAVE to recharge the heat pump.

I had a gauge set (with 2 of 3 hoses still good), and a couple 16oz canisters of propane.  Fuel-grade propane can have moisture in it which is supposedly bad for a heat pump.  Instead of trying to buy refrigerant-grade propane (r-290), I decided to run the propane through a drier.  I bought a drier with 3/8 copper sweat connections and a shrader valve at Wolseley (about $20 total).  I bought a propane torch and unscrewed the tip.  Here's my parts:

I cut the 1/4" copper tube off the schrader, then soldered it all together:
I hooked it up with a propane tank to my gauge set to check the pressure.  The pressure was slow coming up, probably because the pinhole orifice in the propane torch was too small.  I unsoldered the tip, drilled the pinhole out to 1/16", and then tested it to see how much more propane comes out:
I soldered my rig back together, and then hooked it up to my heat pump and started charging it from the low side.  After a few minutes I turned on the heat pump, and was reading ~30psig on low side and the suction tube temperature about an inch away from the compressor was ~5C.  That was about 20C of superheat - much higher than what it should be for optimal efficiency.  After a few more minutes I had gone through about 400g of propane and my low side pressure was 40psig, with the suction tube temperature around 0C.  The antifreeze mix coming out of the heat pump was -8C; right around the evaporation temperature of propane at 40psig.  I'll do some more performance measurements later; for now the heat pump is working OK.

Wednesday, December 5, 2012

Heating cost comparisons

What is the cheapest way to heat your home?  I'll crunch the numbers for a house in Nova Scotia.

Electric baseboard heaters are cheap to install, but expensive to use.  The current cost of electricity in NS is 14.6c/kWh tax in.  With one kWh of electricity providing 3412 BTU of heat, electric heating costs 4.28c/kBTU.

If you use the time-of-day tariff,  off-peak electricity costs 8.15c/kWh tax in.  That reduces electric heating costs at night and weekends to 2.39c/kBTU.

Furnace oil is selling for $1/L, and provides ~36kBTU when burned.  With a 90% efficient condensing boiler, the cost is 3.09c/kBTU.

Discount propane sells for ~60c/L at Costco, and provides ~26kBTU when burned.  With a 90% efficient condensing boiler, the cost is 2.56c/kBTU.

Wood pellets provide ~8kBTU/lb, and sell for ~$5 for a 40lb bag.  At 90% efficiency, the cost of heat is 1.74c/kBTU.

Heating with wood pellets is pretty cheap, but still not the cheapest.  A geothermal heating system will have a COP of at least 3.0 (4.0 can be achieved with new high-efficiency heat pumps).  Take the 4.28c/kBTU cost of electricity and divide by the COP (3.0) to get a heating cost of only 1.43c/kBTU.  With a time-of-day tariff and off-peak use, the rate is just 0.8c/kBTU.

Don't forget the cheapest (free) source of heat - the sun.  So on those sunny winter days, open the curtains, raise the blinds, and let the sun shine in!

Friday, November 11, 2011

Eco Cars

Canadians will soon be able to buy all-electric vehicles.  I decided to compare one to the hybrid and an inexpensive car with good fuel economy.  I'll calculate the cost of 20,000km of highway driving with gasoline costing $1.25/L.  Vehicle prices are SRP.

The Mitsubishi i-MIEV will sell for $33K, at a claimed 1c/km for electricity.  Total driving cost: $200.

The Prius is $28K and has a fuel economy of 4.0L/100km.  Total driving cost: $1000.

The Kia Rio 5 is $14K and has a fuel economy of 4.9L/100km.  Total driving cost: $1225.

Dividing the cost of the vehicle over 10 years (excluding financing & maintenance costs) puts the Rio5 on top:
Prius:  $3800/yr
i-MIEV: $3500/yr
Rio5: $2660/yr

I often drive a motorcycle in good weather, so for fun I calculated the annual cost for a Honda Shadow 750: $1975.  A smaller CBR125R: $1050.