16-10-2012, 11:40 AM
Net-Zero Energy Homes:The Basics of What You Need to Know
Net-Zero Energy Homes.ppt (Size: 6.4 MB / Downloads: 218)
Definitions
NZE: A building that produces as much energy in a typical year as it consumes.
Consumes grid power when it needs it
Feed power to grid when it has extra
ALL energy considered
Electric is not special.
NOT Zero Carbon, or Zero GHG
NOT off-grid
Much more difficult
NZE Design Targets
Produce as much as we consume
Production is usually MUCH more expensive than reducing waste (efficiency/conservation)
Hence the energy demanded by building should always be reduced, reduced, reduced before adding production
Check cost of reducing demand vs cost if supplying energy
Takeaway Lessons
Conservation measures first! Good design/orientation, good enclosure (shell), good mechanicals
Then start adding renewable energy
Insulation has diminishing returns
Renewables can be more cost-effective than insulation after a point!
Net zero energy: good & noble target, but out beyond “neutral cost”
Energy Supply
Renewable energy (RE) or cleaner energy (CE)
Net Zero currently demands site production
This eliminates some good economical RE
Common choices
Photovoltaic: Electricity
Solar thermal Warm / Hotwater
Combined heat and power
Wind electricity
Energy Supply (RE)
PV
Straightforward installation, easy to predict output
Expensive but electricity is very useful and excess can easily be sent to the grid (grid=battery)
Rated by peak output under standard solar conditions (“peak Watt” or Wp)
Costs now $8/Wp (before subsidy) installed
Solar thermal
Intermittent source of hot water
Well developed
Requires big storage tanks in most application
Freezing, over heating, glycol thickening failures, and low temperature efficiency are issues
Not the most economically-viable choice ($6-10 K): but if going to net zero…
Furnaces
Condensing gas furnaces: 90%+ AFUE—mature technology
Sealed combustion
ECM motors (“variable speed”) reduces fan electrical energy
Ground-source heat pumps
Uses constant ground temperature to provide heating & cooling
Fluid pumped through underground tubes; heat extracted or rejected
One of the highest efficiency space conditioning systems (measured ~3.5 COP)
But….
Installed cost of system very high (drilling ground loops)
Nameplate efficiency < actual efficiency (previous example: 5 COP rated number)
Pumping energy
Systems with problems—difficult to diagnose, expensive to fix
Can still suffer from normal ductwork-based system problems
For small loads, is it worthwhile?
Need for Ventilation
Greater airtightness for energy reasons, in net zero houses
Also improves sound, odor, pest control
But people still stink! (+ activities)
Controlled mechanical ventilation
Point source control (exhaust fans)
General dilution ventilation
Indoor Air Quality
Pollutant production
Pollutant removal
Dynamic Balance= pollutant level
Not a IAQ problem if it is not in the air
Solutions
Reduce pollutant production
Increase pollutant removal
Ventilation
Given sensible source control, constant ventilation can dilute pollutants to a low level
Ventilation rates are mostly about odor and humidity, not oxygen
7.5 cfm/person + 0.01 cfm / sq ft
Commercial and highrise 15 cfm/person (!)
Mixing is necessary or separate supply to each room to achieve best IAQ
Exhaust pros and cons
Lowest cost installed system (typical), but problems associated:
Carbon monoxide alarms
Lack of filtration
Dust marking on light carpets
Dirt/grit particles settling on horizontal surfaces
Lack of distribution
Moisture accumulation and odor buildup in rooms remote from exhaust fan
Objections to fan noise
Supplemental Humidity Control
Good energy efficient design reduces sensible cooling loads—insulation, good windows, airtightness
Latent load remains the same!
Thermostat (temperature control) → humidity is not controlled
Need supplemental dehumidification in hot-humid and mixed-humid climates (high performance houses)
Demonstrated in 20 research houses
Case Study House (Westford Habitat for Humanity)
Based on recently-built house
Super-insulated enclosure
Very airtight (1.5 ACH 50)
Best-in-class mechanical systems
Energy Star appliances
Compact fluorescent lighting
No renewable energy added: not NZE (PVs or solar DHW)