Under
current circumstances economic payback for a micro-scale wind turbine - a long
term investment - may not be achieved for more than ten years. The
- Capital outlay
- Demand
for electricity and the timing of the demand
- Load factor
- Maintenance costs
- Average wind speeds
- Turbine performance
- Cost of
alternative sources of power
- Life-time
of the turbine and associated equipment
- Value
placed by prospective buyer on Building Energy Rating
- Payment
or credit for excess electricity exported.
Capital Outlay
Quality
turbines of any scale are not cheap. A good quality 5kW - 6kW turbine can cost
over €25,000 to purchase, install and connect. A well maintained good quality
6kW unit in an excellent site will produce around 13,000 units of electricity
per annum. Depending on the proportion of this power which is consumed onsite,
exported for 9-19c/kWh or exported for free it has a value from anywhere
between €1300 and €2400 to a domestic customer of ESB Customer Supply.
13,000 kWh
would have the value of around €2400 if all of the units produced were consumed
onsite and the site was supplied by ESB CS with the 24-hour rate (rather than
the night-saver). However a typical house in Ireland might consume around 5,500
units of electricity per annum and not 13,000. The electricity generated would
have the value of €1760 if the turbine matched demand for 3,000 kWh of a
typical houses demand (at 18.6c/kWh per unit) and exported the balance (3,000
kWh at 19c/kWh and 7,000 kWh at 9c/kWh). The house would then still have to import
2,500 kWh at 18.6c/kWh (or €465 worth of electricity).
Demand for power and the timing of demand
Clearly the
proportion of power consumed/exported/imported can vary widely from premises to
premises. Customers could monitor their own energy use for a period of months
to get an idea of the timing of their demand. A variety of DIY energy monitors
are available and the data collected can be uploaded to a computer for
analysis.
Well sited
consumers who remain high demand customers after employing all appropriate
energy efficiency measures may be best placed to benefit most from the addition
of microgeneration. Wind microgeneration is suitable to sites where there is a
constant 'base-load' demand for electricity. That is where demand rarely drops
below a certain level, even at 5 a.m. If a turbine is installed it can displace
imported power required to meet the base-load demand. It should however be
remembered that 'night-saver' or other economy tariffs are available and these
should be included as appropriate when analysing the economics of the site.
Load factor
Every demand
customer has a 'load factor. In electrical terms a load factor can be expressed
as the ratio of the maximum electrical demand versus the average demand over
the same period (peak v. average). A rugby club with floodlights may have a
very small demand for 95% of the week. For a few hours every week during 6
months of the year the demand reaches its peak when floodlights are turned on
for training. Installing a wind turbine on a site such as this is far from
ideal from a demand and cost reduction point of view unless an appropriate
export tariff can be availed of.
Maintenance
costs
Maintenance
costs and contracts should be discussed with a number of prospective suppliers.
A routine maintenance schedule should be supplied by the manufacturer as well
as estimates for future costs of major parts. The schedule should be sufficient
to ensure long and productive operation under local conditions. Some turbines
require more maintenance than the manufacturer recommends while others may
survive with less. As is the case with all machinery, the more frequent the
maintenance and care the longer the turbine is likely to be of use. The
ultimate aim is to reach beyond the break-even point. A turbine system which
does not reach a break-even point has in effect produced power during its
lifetime that has been more expensive than would have otherwise been available
from the grid.
Maintenance
costs usually include a call out charge as well as costs to cover access to the
turbine for greasing and other light maintenance. After a period of years parts
such as blades or brushes may need to be replaced. Estimates for the lifespan
of such parts and the cost of such non-routine maintenance should be provided
by the supplier or manufacturer. Again local conditions, turbulent air flow or
storm damage may require replacement of parts outside of any schedule.
The more
mechanically and electronically complex the turbine is the more elements there
are that can go wrong, therefore the more maintenance which may be required. Simplicity should be a factor in choosing a turbine.
Average
wind speeds
The output
of a wind turbine is dependent on the energy in the air flowing over the blades
and through their swept area. Power output for a given turbine is proportional
to the cube of the wind speed i.e. if the wind speed doubles the power output
increases by a factor of 8. Another way to look at it is that a doubling of the
power output can be achieved by an increase in the wind speed of just 25%. So
it is clear that the power output and its value can vary substantially from
site to site. Average wind speeds are more important than the occasional high
wind speeds which might be available at a site.
Turbine
performance
Manufacturers
and suppliers should supply all of the technical specifications and expected
performance figures for the turbine and system. Each turbine has a power curve
which will give an indication of the turbines output at different wind speeds.
A power curve which an accredited third party test or independent test facility
has developed is more reliable than a manufacturer supplied one. Customers
should always check the accreditation of test facilities providing
certification. A power curve will show information such as cut-in wind speed,
peak power, rated power, rated wind speed and cut-out wind speed.
A low cut-in wind speed is desirable to capture as much available energy as
possible. At cut-in the turbine will produce little power but it is better to
be producing something rather than nothing. Before cut-in turbines may be
rotating but the generator is not producing any power. A typical cut-in wind
speed for small turbines is 3m/s.
Peak power
is the maximum the turbine will produce in infrequent circumstances such as
exceptionally high winds or momentary gusts. Over-speed controls should prevent
too much power being conducted through the electronics but peak power could
cause damage if the turbine is not designed or prepared to respond quickly or
if the mechanism fails. The electronic controller and inverter are designed to
cope with currents and voltages within a range and a momentary breach of upper
limits will damage or burn out components, if not the entire unit. The inverter
must be sized appropriately to cope with peak power production.
The key
performance indicator for a wind turbine is the amount of energy it produces
over its lifetime and not the rated power/nameplate rating.
Cost of alternative sources of power
The power
produced will replace imported electricity at the retail rate available at the
moment it is produced. At present domestic customers generally have access to
just two types of tariffs. One account has a flat tariff throughout the day and
night. The other option is a day/night tariff with the meter differentiating
between units consumed at night and during the day (ESB's 'Nightsaver' product
for example). The day-time tariff in the day/night option is slightly more
(6.8% more) than the standard 24 hour tariff but the night-time tariff is
significantly less (47% less). The day/night tariff option is suited to homes
with electrical storage heating.
Commercial
and residential commercial customers have a range of tariff suites available
from a choice of suppliers. Anyone, commercial or domestic customers, considering
their tariff options need to consider carefully their consumption patterns and
factor in extra standing charges applicable for the required metering. There is
also a charge for the required meter.
Some
microgeneration sites may chose to use the power produced onsite to heat water,
be it for washing or for space heating -direct water heating. In this case the
fuel cost comparison is carried out against available heat fuel sources such as
oil, natural gas and biomass boilers or stoves. For situations where just the
excess electricity is used to heat water it is just that portion which is
compared to other heat sources with the remainder compared to electricity
imports as before.
Life-time of the turbine and associated equipment
By
definition a turbine that lasts beyond its break even point or payback period
has paid for itself. It is also clear that a turbine that only lasts a small
few years is a wasted investment. Turbines need maintenance and it is in the
interest of the owner to keep the turbine generating for as long and as
efficiently as possible to get to a point where the turbine has paid for itself
and it is producing clean energy at a cost less than the retail price of
electricity.
Value placed by prospective buyer on Building Energy
Rating
Adding
microgeneration to a property will improve its rating under the Building Energy
Rating (BER) requirements. The output from electricity producing photo-voltaic
(PV) panels will be estimated as per a prescribed formula. For wind turbines
there must be at least one year's output data before it can be factored into
the buildings performance. This is to account for a wide variation in
performance of turbines- from good sites to poor sites and from maintained
turbines to defective ones.
Prospective
buyers, leasers of a premises or tenants will place a value on the energy
rating of a building. Running costs can be a key consideration when valuing,
purchasing or renting a new property.
Payment or credit for excess electricity
Payment by
electricity suppliers willing to buy excess electricity will reduce the payback
period and improve the viability of the technology. With a payment of between
9c and 19c/kWh being offered by some of the ESB Group every unit of electricity
produced has a value and units will no longer be exported to the grid for free.
To give a
simplified example: A house in an excellent site for availing of wind has a 3kW
turbine installed. The turbine produces around 6,300 units of electricity
(kWh). The house has an annual demand of 5,000 kWh but the turbine only matches
this demand for 40% of the time i.e. 2,000 kWh of the house demand comes from
the turbine. The remaining 3,000 kWh comes from the grid.
If no export
tariff was available the turbine output would have the value of €372 (retail
price of electricity (e.g. 18.6c) x 2,000) as the amount exported (4,300 kWh)
would have no value. With payment for export available to domestic customers
each of the 4,300 units now has a value. The first 3,000 kWh has a value of
19c/kWh (equal to €570) and the remaining 1,300 kWh has a value of 9c/kWh
(equal to €117). Thus the payback period is reduced by earning €687 from
exports annually. This export payment is on top of the payback accruing from
displaced imports (the 2,000 kWh mentioned above).
