Friday, 30 September 2011

Please don’t raise the speed limit to 80mph!

There’s talk in the press today of raising the speed limit here in the UK.  While admittedly most people seem to drive at 80 anyway, increasing the limit will definitely affect most people’s pocketbooks.  I also feel it will increase pressure on other people to drive faster.  If the limit is raised, perhaps it should become an absolute limit, removing any margin of doubt.

This summer, we drove to Spain fully loaded with a roof box on top of the car with my wife and two children.  A 2500 mile marathon!


On the journey down through France, we drove at just under the speed limit of 130 kph (~ 80 mph).

We don’t do a lot of driving long distance in the UK, but specifically bought our Kia diesel for long trips like this one, and carting my daughter’s sailboat around the country to various events, and we use the cruise control a lot.

The car is fairly efficient, having averaged 46 mpg with a boat on top.  On the journey to Spain, we averaged only 44 mpg, with some legs in France getting as low as  38 mpg.  Once in Spain at 120 kph and the mountains of the Pyrenees, the economy rose, bringing the average up.

You’d think that would be the end of the story, but on the way home we made another change.  We bought wine; a lot of wine!  About 60 litres in Spain, and then added another 30 in Calais.

When we stopped for dinner, I realized my tyres were looking a bit low, so I dug out the manual while crossing in the tunnel, and decided that they were probably pretty underinflated.

Don’t forget to keep your tyres inflated properly

I decided that despite the very late return home, we had better get some air, so stopped at the exit of the tunnel, and inflated them to the recommended level for the load.

What a difference.  On the journey back through Spain and France, we managed to average only 39.5 mpg.  That’s 11% down on the journey down, just because I didn’t inflate my tyres properly.

To complete the exercise, we drove home in the car in the same state, but this time stayed at 60 mph, as it was very late at night, and we were tired, and the car was very loaded.

Having filled up in Calais, I had a new baseline, and this time with the same full load and roof box, we managed 49 mpg.

You can trade time for money

It looks like driving 20 mph faster decreases fuel economy by 11% as well.  And driving additionally with underinflated tyres decreases by a total of 20%!  I’m lucky the fuel averaged only £1.10 a litre on the continent instead of the £1.40 pretty much balancing out the extra unnecessary expense.

Further mileage tests

I also had the occasion to do two other long trips recently.  To London in the C1, where driving at 60-65 on the motorway increased mileage from the 47 mpg my wife got to my 55 mpg, again a 15% decrease for a 10 mph change.

Finally, I drove to Southampton with 4 people in the car and no roof box.  We had a good journey from Cambridge, and again, drove at between 65 and 70 mph.  This time, we managed to achieve 58 mpg, a record for the car, showing that adding weight and a roof box appears to decrease fuel economy by around 11%.

Thursday, 29 September 2011

It’s Indian Summer here in the UK, 30°C/86°F here today!

The last two days (September 28, 29) we’ve had cloudless skies, so sun all day long.  We also had a very good day on September 23, but the temperatures were much cooler (around 17°C), and there was a bit more cloud.

What better way to find out how temperature affects solar PV output!

Though the total on each of the days was around 9kWh, with the peak both yesterday and today at 1.5 kW, but on the 23rd, we had our record output for a 5 minute period of 1.8 kW!  So it looks like a warm sunny day will reduce output by about 17%. 

The system is 2.1 kWp, and given the sun is pretty much over the equator on the 23rd, we can calculate that at noon on that date, the sun is at 38° (90° – 52°) elevation, and the roof is at 30° elevation, making an angle of incidence of 58°.

Sin(68°) * 2.1 = 1.94, so the panels seemed to be within spec on the 23rd.

Today, the sun was already down to 35° at noon,  therefore
Sin(65) * 2.1 = 1.90, so the difference in solar radiation was 2.1%.

Ouch—15% degredation with temperature!  I wonder if there’s any way to cool the panels more effectively?

Current Cost EnviR shortcomings regarding Power Factor

We recently installed a Current Cost meter

The system allows you to clamp a sensor around your mains cable which broadcasts readings of current to the display.  It also allows connection of up to 9 appliances.  The display aggregates the data into two hourly, daily, and monthly bins.  It also transmits the data on an RS-232 to any listening PC.

The main sensor clamps onto the live output to the main fuse box in the house, which measures all current being consumed by the house.

Our challenge was to try to use this data to monitor where our energy consumption was going, as well as collect information on our solar PV installation.

In order to monitor additional systems in the house I bought a pack of three of the sensors on the left.  One clamps around the live feed from the Solar PV system, measuring Generation.

I clamped another around the live leaving the fuse box going to the underfloor heating system.

The final sensor was intended to be clamped around the mains cable to the cooker.

Current clamps work with a single core only

Unfortunately, my strategy of clamping around a cable in the loft to measure the current does not work.  You must clamp it around only the live cable, or the current on the neutral cancels out the live and you get no reading.

Because I had an extra transmitter, I made a special extension cable where the live passes through the sensor, so I can plug in any set of appliances into this cable.

What’s using all that “power”

When my system eventually was up and running after the initial set backs, and some opening of my fuse box to get the clamps on the circuits, I noticed that my under-floor heating was reporting 35 watts when it was not actually heating.  This struck me as too much.  I wouldn’t leave a light bulb on all the time.  While the thermostats were warm, they weren’t that warm.

So I powered down the entire house, switched on the under-floor circuit only, and found that they were not using any measurable amount of power.

Power Factor strikes!  This happens in several circumstances.  Large magnetic loads (refrigerators, and pretty much anything with motors) only use a portion of the power, returning some of it back out of phase.  The link above tells you more about it.

Now, it’s really hard to monitor “power”

Unfortunately, this makes life difficult as the sensors are describing power that is not actually being billed to you (though it does cause inefficiencies for the power companies, and the amperage actually does flow through the circuits in the house, increasing load).

What else suffers from this? My PC for one appears to show a power factor of 0.7, which means that I have to multiply the reading on my meter by this factor to get real power.

Oh, and the Inverter for the Solar PV as well.  It shows 80 watts at night, supposedly consuming this amount, though in fact it only consumes less than 1W.

There is a solution for individual appliances.   These measure the real power on the device.  Unfortunately they don’t also let you know the power factor, so as a result, you cannot subtract the “phantom” consumption from the total.

They do though let you measure real cost for the individual appliance.

Note that in the UK, consumers are not billed for Power Factor.

Dimmer switches have no effect?

I also tried measuring what happened when I used the dimmer switches on eight 50W halogen lamps.  Apparently nothing, they use almost exactly the same power. 

Of course, that can’t be right, as the dimmer switch isn’t acting like a 500W heater when the lights are dimmed.

While researching the new LED lights I talked about yesterday I was warned by my electrical supplier that they’d had some problems dimming them.

Philips have a document that describes the way these things actually work.  Again, you can see what’s going on with the power factor, as it’s only consuming a portion of the load.

Too many Sensors?

So, now I’ve populated my system with a main meter, 3 additional clamps an an appliance monitor.  The system is supposed to be able to handle 10 inputs with up to 3 readings on each.

Unfortunately, it drops packets as they are colliding, and not being received by the main unit.  There is no protocol to avoid collision.  So we now have lumpy sampling to deal with as well.

Buyer Beware

I hope this has been a useful overview of the shortcomings of the Current Cost system, which while flexible isn’t really good enough for highly accurate measurements, due to the power factor issues, and lost packets.

We’re doing our best to produce some software that alleviates some of these issues, so stay tuned.

Wednesday, 28 September 2011

New energy saving gadgets ordered

Following on from today’s earlier post, I’m still searching for ways to make the house more efficient.


We currently have some 240V, 50W halogen lighting in the hall, which generate 1000 candelas of light.  Now, there are 8 of these, so that’s 400W when they’re all switched on.

Today I ordered two Philips Master LEDlamps (these things aren’t cheap—over £25 each including VAT), but they do only draw 7W.  They’re also dimmable, and should fit in to the existing sockets.  If they don’t work out, I can still return them and get my money back.


In addition, I also ordered two electronic radiator control valves, that allow me to programmatically set the temperature a room should be at various times.  The target is the living room with the gas fire.  While the balanced flue fire appears to be around 90% efficient, it’s still easier to heat the lounge with the main central heating.

The plan is to not heat the room at all until 3PM, and then heat it to a base temperature until 8:30pm-ish, when the heating will switch off.  Normally, if we’re in the room, we’ll put on the fire to make it extra cosy, so if no-one’s in for any evening, it won’t keep heating.  When the fire’s on, it will warm the room sufficiently for the boiler to stop supplying heat to that room, and heat the rest of the house instead.

These devices aren’t cheap either (£25 each), and I need to drain the central heating system to add them, but as the current valves are leaking, I need to get a plumber in anyway.

Fingers crossed.  I can’t wait to see what effect these things have.

Solar PV, 3 weeks in

Our solar panels have now been in for exactly 3 weeks, and in that time we’ve generated 140 kWh of electricity.

It’s been a fairly sunny three weeks, but the panels are generating to specification.  If you want to know more, please contact me.

In order to see what is going on, I also purchased an energy monitor, which I’m developing some software for. The monitor measures the current every 6 seconds; measuring what the house is consuming, what the solar PV is generating, and aggregates the data. You can see our usage since I bought the monitor above. I’ve also got sensors on several appliances around the house as well as the under-floor heating, so more measurements should be coming as we enter the heating season in October.


The green dots at the top are the total kWh used by the house each day, and the yellow dots at the bottom are what was generated by the solar panels.   The bar graph is the consumption for two hour periods each day, the the sample immediately after the date line being from 1-3 AM.  The values above 0 are consumption, and the values below are generation from the solar panels.

The monitored values are currently inaccurate, as you’ll notice the generation in the middle of the night.  This is because the current measurement does not correct for power factor, but that is alas another blog post.

The good news though?  Over the last 3 weeks we’ve used almost no net power during daylight hours, averaging just 1.5 kWh.