by Peter Fairhurst

Part 2 - Wind and Solar Power

In Part 1 (published in the Spring edition of Pipeline) we debated the pros and cons of using either the boat’s engine driven alternator or a portable generator to charge the boat’s domestic batteries whilst away from the home mooring. In Part 2 we shall now consider the use of either wind or solar energy (green power) to charge the batteries and highlight the limitations of both systems.

Wind Energy

Wind energy is converted into electricity using a small wind turbine (not a wind generator – that is a tin of baked beans) directly coupled to an electrical generator. The turbine is mounted on a mast to limit surrounding obstructions causing disturbance to the wind flow onto the turbine blades that would result in a reduction in the potential output from the generator.

Because the output from the turbine is not constant, due to variations in the wind velocity, the turbine has to include an electronic regulator unit to ensure that a constant and correct voltage (with varying current) is available at the battery terminals. These micro turbines have fixed pitch blades unlike large industrial wind turbines that have variable pitch blades capable of adjusting to the wind velocity (as measured by a wind anemometer that forms part of the turbine control system) to maximise the power output.

The obvious limiting factor in the use of the wind turbine to charge batteries is that if there is no wind there is no power to be generated – not much use after a still period in the middle of summer with the fridge on all the time, the lights on till late and other sundry electrical items in use whilst moored for a few days in that delightful sheltered spot you have found.

However what happens when there is a gale blowing? For reasons of safety there is a maximum rotational speed at which the turbine will generate power so that there is a limiting wind velocity above which no additional power will be generated.

As an example of the type of wind turbine-generator to be considered we shall look at the power curve for the Rutland 913 turbine which shows that the electrical output is at a maximum when the wind velocity is 38 knots and this corresponds to an output of 18 Amps @ nominal12Volts.

Reference to the UK chart for average wind speed at 25 metres above ground level shows that for most of the UK the average wind speed is between 5 and 8 metres per second corresponding to between 9.7 and 15.6 Knots.

This average wind speed will produce a generator output of 1.5 to 3.5 Amps that would be useful as a trickle charge but would not recharge the domestic batteries quickly after a heavy discharge.

The maximum output is of course only available if there is enough wind to power the turbine so if the wind doesn’t blow the Watts don’t flow! I have used the Rutland 913 wind turbine as an example because the physical size of the unit is about as large as can be reasonably handled if it is decided to carry it on the boat whilst cruising. The turbine tail fin that automatically maintains the turbine pointing into the wind as this changes direction can be a nuisance when low bridges are encountered.

Noise from the turbine is not considered to be excessive by most people that I have contacted however, this a subjective issue that only an individual can decide upon. Reliability of the wind turbine is excellent as the rotating elements are simple and robust with long term maintenance limited to bearings and brushes replacement. The cost consideration for this type of equipment is quite a high initial cost at the outset but with few running costs thereafter and a long equipment life.

Solar Panels

Solar (PV - photo voltaic) panels, work by converting light (not just sunshine) into electricity using photovoltaic cells. The intensity of the light will determine the output from the panel and it therefore follows that during dull overcast days the electrical output will be reduced and similarly the shorter daylight hours in winter will reduce the capability of the panels.

The electrical capability of the panels is determined by the number of ‘cells’ that comprise the panel in the same manner that a battery output is a function of the number of cells.

Each cell in the solar panel produces a mV output and when these are connected in series produce a line output of 12Volts – (for our purposes, although different voltages are available). A number of these series lines are then connected in parallel to increase the wattage available from the panel.

Siting the PV panels on the roof of the boat would not endear the boat’s appearance to the purist, but if the boat is left unattended for long periods these panels could provide the answer to the problem of battery drainage. A charging controller prevents the batteries from becoming overcharged.

In order to maximise the benefits of the PV panels they should be angled and rotated to capture maximum light. The performance of the panels also deteriorates with time (ageing) with the panels losing about 1% of their capability per year.

As there are no moving parts in the PV panels, longevity should be inherent and for the most part it is. Manufacturers typically offer guarantee periods of 10 to 15 years (or more) against performance failure of the cells - subject to normal use. PV panels are used at sea to power lightbuoys so their capabilities are proven and accepted by various marine authorities.

Conclusions

Well, what can we conclude from the information contained in these comparisons of the various forms of battery charging.

The least initial cost option (it comes with the boat) is to use the boat’s own engine/alternator equipment with a battery management system that will charge the domestic batteries in a reasonable time. The downside to the regular use of this method is that it will lead to engine fouling and premature wear of the engine. Remember, the principal purpose of the diesel engine is to propel the boat and not to charge the batteries or heat domestic water.

The petrol engine driven portable generator is also a flexible option with the added advantage that low power mains appliances, such as power tools, lighting etc, can also be used if no other mains power supplies are available. To take full advantage of the portable generator mains output it is necessary to combine it with a battery charger that you may already have installed if your home mooring has a mains power supply.

Maintenance requirements for a portable generator are usually confined to changing the oil (and filter if fitted – most don’t) and replacement of the spark plug so that over a 12month period the costs are not high.

The downside to this method is finding a safe and sensible place to stow the generator and spare petrol. Those with a cruiser stern have the advantage that they can build the necessary weatherproof and ventilated enclosure as part of the rear deck seating arrangements.

The initial cost of a portable generator of around 1kW output is not cheap but, with careful use, it should last for many years.

Wind turbines provide a limited source of power for battery charging and can also be transported with the boat when out cruising. I have seen boats that had brackets attached to the rear cabin bulkhead thus enabling the turbine mast to be easily stepped and secured for use when moored. If the wind is blowing, within the acceptable velocity range, then a steady charge will be available with no fuel costs to consider and possibly supplemented during daylight hours by the use of solar panels.

The downside to the use of a wind turbine is that if the wind doesn’t blow you cannot charge the batteries and so an alternative charging source (the boat’s engine?) must be available. It would seem that the wind turbine option is better suited to the home mooring where it can be left to provide a relatively small charge over a long period to maintain the batteries at peak charge.

Solar PV panels are undoubtedly useful for providing a source of low power dc energy to allow the batteries to be trickle charged possibly in conjunction with a wind turbine. A visit to the Sharp Electronics (UK) Ltd website gives details of more low voltage applications for PV panels.

During this years boat festival at Loughborough, I spoke to the recent Owners of HODI (non-Piper boat) who were living on their boat and they had both a wind turbine and solar panels for battery charging. Their limited experiences with this equipment indicated that whilst the system worked well it was only able to provide a fraction of their usage (under most weather conditions) and they had to resort to running the main engine to complete the battery charging.

Comparing the Costs

The least cost option is the boat’s own alternator but if used regularly for battery charging it comes with the associated long term problems highlighted earlier.

The next most economical option is the solar (PV) panel (depending upon the number) but with the limited output. Typically an 80 Watt panel array will cost £300. plus the charge controller

The next option would be the wind turbine (Rutland 913) with a cost for the full package (turbine generator, control box and mounting kit) of around £450

The portable petrol generator can cost either more or less than the wind turbine as an example the Honda Eu10i costs around £630 whereas the near equivalent Clarke generator costs around £300.

The purpose of this article is to provide an insight into what can be obtained and to provide some food for thought. As I said in the first article it is not a prescriptive essay and no doubt the costs quoted above can be lowered but as a comparison they illustrate the relative cost.

What do I use? I have a small portable Honda generator (900 Watt) that is used in conjunction with a built-in battery charger on the boat giving a maximum charge of 40Amps at 12 Volts (2.08Amps + losses at 230V). This I find works well and has the added advantage of allowing the use of small mains powered equipment such as tools. The downside to this method is that as the batteries become fully charged the generator is hardly doing any work and so the fuel efficiency is correspondingly low.

May your lights stay bright and your fridges cool!                          

Peter Fairhurst - Winter 2005