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Zero Net Energy

Star How to have no energy bill. 

 

What does ZNE mean?

The term Zero Net Energy often gets shortened to ZNE.

It means that you generate enough energy over the year to provide all the energy you need over the year.  If your house is super insulated and air tight then you will not need much energy to heat it so you will not have to generate much energy to get to Zero Net Energy.

Houses currently biggest energy hogs

Currently in the USA, the houses are responsible for 25% of the energy that the country consumes.

Photo Voltaic (PV) panels

Using Photo Voltaic (PV) panels is the most obvious way to generate power but there are other alternative energy sources too.

Government incentives

You may be lucky enough to have a government incentive in your area to help with the capital outlay necessary to purchase PV panels.

Climate zones

What you need to do to get to ZNE is different depending on where you live.  If you are in a cold climate then you need to reduce your heating requirements and if you are in a hot climate you need to reduce your air conditioning requirements.

 

Almost all energy goes to heat

Almost all the energy you use (eg from the electricity or gas company) gets turned into heat.  Other things energy gets turned into are light and motion but these are insignificant compared with the amount that gets turned into heat.

If the energy usage is with a super insulated building enclosure and the weather outside requires you to heat your house then using appliances within your house is not a problem because the energy used by the appliances is reducing the energy that your heating system needs to use.  It is much better to use your TV to heat your house because you get the additional benefit of entertainment as well as heat.

 

Energy storage

Energy storage is a big problem.  The time when you are most able to generate energy (eg midday using PV panels) is when you have the least energy requirement.  You want to generate power during the day and use it in the evening.  That means you need energy storage.

Electricity company energy storage

You may in your area have an electricity company that is willing to buy the excess energy you produce.  When you generate more than you need at any particular time of the day then your electricity meter runs backwards.  At night when the sun isn't shining then your electricity meter will turn the normal way.  Zero Net Energy equates to the meter on average being stationary.  The electricity company is acting as an energy storage system for you.

Battery storage

This would be an option if batteries were not so expensive and they did not wear out so quickly.

Pumping water

If you live on a large sloped plot then you could use your generated electricity during the day to pump water from a pond at the bottom to a pond at the top and then run a micro-hydro generator in the evening to generate electricity.  Unfortunately it is hard to make the various inefficiencies work out.

 

The building enclosure

Insulation

It is not practical to be ZNE if you don't have really good house insulation.  Good insulation is needed in cold climates to keep the heat inside your house and in hot climates it is necessary to keep the heat out.

Air tight

You need to stop all the air leaks.  Your building enclosure needs to be air tight.  Warm air leaking out from your house is just as bad as not having proper insulation.

The obvious question is how do people breath in an air tight house.  The answer is that you need a Heat Recovery Ventilator .  This sucks out the bad air and uses the heat from the bad air to warm the fresh air that it blows into the house.  They are remarkably efficient at doing the heat transfer so the heat lost from the building enclosure is small.

External doors

These need to be air tight and well insulated.

Windows

Windows are transparent sections of wall and walls need insulation to stop heat loss.  You will not be able to get the R value as good as the R value of the walls, but you need to try to make them not too bad.

Using argon filled double glazed windows it not too expensive and is a good way to get to a reasonable R value.  Triple glazed windows have a higher R value but are too expensive.  Pseudo triple glazed windows that use a plastic sheet in the middle can cut down on the visibility through the window so I am not a big fan.

Make sure you fully seal all round the windows because any air leakage will have a very significant effect on your energy bill.  Don't listen to anyone that says you should not fully secure the bottom nail fin so that water can escape.

Thermal mass

Thermal mass is described here .

As discussed below, it works particularly well when used as part of a Passive Solar system.

The cost of electrical energy can vary depending on the time of day.  It can be less expensive at night so you can use this to heat your thermal mass in the early morning and have the thermal mass release the energy gradually during the day.  This is sometimes referred to as a night storage heater.

Passive solar

Shading is necessary to avoid excessive heat from the sun high in the sky in the summer (which would require energy to power an air conditioner) and yet get as much heat from the low in the sky sun in the winter.

You can fit motorized awnings to shade the sun in the summer, but it's best to design the house properly before you build it.  There should be a decent sized roof overhang.  Balconies above doors and windows is a great way to shade lower floors.  You do have to design the balconies correctly so they are not heat fins radiating heat to the outside world.

You ideally want windows with a high solar gain.  These are actually quite hard to find because most windows are designed for houses that do not have properly designed shading overhangs.

LED lighting

In winter it is not a problem having incandescent light bulbs inside your house that generate lots of heat because the heat in within the building enclosure so it reducing the amount of energy your heating system needs to use.  In summer however you do not want that heat so you need LED bulbs.

Outside lights should always be LED otherwise all you are doing is paying for energy to heat the planet.  Even when using LED bulbs outside it is good to put them on a motion sensor so they don't accidentally get left on for long periods of time.  Here's what I do for exterior lighting .

Wall surface area

You want the biggest enclosure volume with the least amount of external wall area.  Given that it is not practical to build a spherical house then a cube is the next best shape.  Given that you will want a house that has a wide southerly wall (for enjoyment and passive solar) then you should stick with a simple rectangle rather than having lots of protrusions.

House size

It is true that a small house will be less expensive to heat than a large house (assuming the same amount of wall insulation).  Having said that, if you do super insulate the walls then a large house can be quite efficient.  A relatively small increase in external wall surface area gives a big increase in enclosed volume as long as you stick with a simple rectangle for the house shape.

 

Alternative energy systems

Photo Voltaic Panels

This technology is getting better and less expensive all the time.  That's great but it does mean that if you buy panels now they will soon be outdated and you will have paid more for them now than in the future.  That's just the way it goes, so you just need to bite the bullet and go for it.

The best place to put solar panels is on your roof, but the downside is that currently they look really ugly.  I care about house styling and have lots of land so I am currently more in favor of a separate array.

Given that you need overhangs to make passive solar work properly, one idea is to use PV panels above windows to act as a shade as well as an energy generator.

Geothermal

It's a sensible idea to use the stored energy in the ground to heat your house via a heat pump.  The problem is the large capital outlay to implement such a system together with the ongoing maintenance costs.  The payback time for a well insulated well sealed house is likely to be very long.  I'm not a big fan.

Wind power

Not many places have a reliable steady wind.  If most of the time there is not much wind then you will not generate much power.  If you have a wind storm then it could well wreck the windmill.  Maintenance is a problem because the thing is way up in the air.  I'm not a big fan.

 

Controlling your house

Any heating system needs a good thermostat so it properly maintains the house at the constant temperature you want, eg 70 degrees F.

You don't want to be heating your house while you are away, but you do want to come home to a warm house.  A control system, ideally one you can control remotely over the internet, allows you to achieve this.

When you are at home it is a good idea to at minimum have your heating system on a timer so it turns off after you have gone to sleep and warms the house just before you wake up.  Ideally it is good to make it more intelligent so it can respond to the occasional days when you go to bed late or have to get up early.  See Home Automation.

Adding more insulation to the windows during the evening and night in the form of automated curtains or shutters is a good idea.  When it's dark outside you don't need windows to be transparent.

 

Monitoring your house

It is better to spend money on improving the performance of your house rather than monitoring it to see how badly it is performing.  If you are a bit of a geek then by all means get some monitoring gear so you can brag about how well your house is performing.

Informing yourself about each appliance

An inexpensive way to find out how much electricity a particular appliance is performing is to use a Kill-a-watt plug in meter.  Being conscious about which appliances take the most energy prompts you to use that appliance less often and will save you some energy.  Equally it stops you sitting in the dark because you are trying to save on lighting energy when it is way less significant than using your oven once a day.

Another example is people who suffer the inconvenience of unplugging their TV when not in use rather than use the remote control standby mode.  A TV in standby mode uses hardly any energy and anyway any heat energy produced is within the house envelope so is reducing the energy needed by the heating system.

 

 

 

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