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.

Sunday, April 24, 2011

Saving with timers

Many states and provinces now meter electricity based on time of day, with weekend and night rates near half of the daytime rates.  Besides timers for hot tubs, many other high-use appliances can be timed to run when electricity rates are lower.

Dishwashers with electronic controls typically have timers, and especially if you use added heat or heated dry the timer is worth using.  Some washers and dryers also have timers, and although most people know an electric dryer uses a lot of power, washers also use a significant amount. Using a clamp-on ammeter, I measured the power of a 240V electric dryer on medium heat setting to be 5600W.  Using Nova Scotia's power rates, running the dryer for an hour would cost 73c, but only 39c during off-peak rates.  The washer uses 750W, so using the washer for an hour during off-peak rates would save about 5c.

Hot water heating is the second largest household energy user in Canada after space heating.  When I first switched to TOD rates I installed an Intermatic EH40 timer on the hot water heater.  One year later it failed.  For a replacement I installed an Aube TI040.  I paid less than $100 at Harris & Roome, which is less than I paid for the EH40.  It also has a 3-year warranty vs. 1-year on the Intermatic.  When installing a water heater timer, it's a good time to check your water heater element; many 40-gallon hot water heaters come with two 3000W elements installed, but can accept up to 4500W elements.  A 4500W element will heat 50% more water in a given amount of time than will a 3000W element.

Cost savings will depend on household hot water use.  A 2008 study by NRCAN, "Hot Water Use Field Test Results" indicates a four-person household uses about 190L of 50C hot water per day.  Assuming a cold water supply temperature of 10C, heating 190L of water uses 8.8kWh of electricity.  At normal rates that is $1.15 vs. 62c at off-peak rates.  With savings of 53c/day, the timer will pay for itself in less than 7 months.

Saturday, December 11, 2010

Air infiltration

I think air infiltration doesn't get the attention it should.  Air sealing gives the best heating savings for your dollar when compared to almost every other building envelope improvement; insulating exposed concrete basement walls is the only other thing I can think of that comes close.

I'm pleased to see marketing funds being spent on this issue, like this commercial featuring David Suzuki.

Using Hot2000 to model a house located in Halifax, NS with R26 walls, R40 ceiling insulation, and energy efficient windows, about 27% of the heat loss in January comes from air infiltration.  Hot2000 doesn't count the heat loss due to the moisture in the inside air (latent heat), and recent research indicates it underestimates the total air infiltration rate.  Considering Hot2000 errors, a more accurate estimate would be 40% of the heat loss coming from air infiltration.

Although I think the best way to find air infiltration points in a house is with a blower door test (typically $50-$100), there is a cheaper way.  I read on builditsolar.com the idea to turn on a clothes dryer, then go around your house checking for leakage.  If you have a kitchen range hood exhaust fan and bathroom exhaust fans, turn these on too.  In addition to checking the usual places like around windows and electrical boxes, check for air infiltration along the floor on outside walls.  If you have tile or hardwood floors, a bead of translucent or clear caulking between the baseboard and floor can cut down air infiltration.