SOLAR SPLASH Newsletters

SOLAR SPLASH^® "Handbook"

The three Handbook Chapters contain information from various Solar Splash newsletters and form a "handbook" for teams preparing to enter SOLAR SPLASH for the first time or for those getting ready for their next SPLASH.

Handbook Chapter 1 - So you want to enter the SOLAR SPLASH

Handbook Chapter 3 - Getting the most from the sun

Chapter 2 - Managing the Sun

Over the history of the Solar Splash, much has been written on the subject of energy. From incoming and how it is measured to storage and consumption. Some is in the form of Rules, some as Hotline questions, and some in News stories. The following is an attempt to compile all of this into one chapter of the "Handbook".

Like electric cars, electric boats have been around for a century. Whether a launch or a submarine, the drive systems of these craft are typically simple, reliable, and energy efficient. Although an enormous amount of energy is available in one gallon of gasoline, it is not renewable. On the other hand, most boats are outdoors most of the time and often are not in use for many hours a day. Using that down time to let the sun charge on-board batteries is an excellent use of solar energy. This is why the only energy source for the Solar Splash is direct incoming "Sunlight".

Why 480 watts?

Solar Splash is intended to be a competition that draws attention to a practical application of Solar Energy. It is hoped that having stimulated the interest of many, others will use solar energy to whatever extent is practical from a one square foot panel on a kayak to power a lap top computer to a sight-seeing boat where the entire roof is covered with solar panels.

The first solar car competition, the Tour de Sol, held in Switzerland, developed the rule limiting solar panel output to 480 watts under one-sun conditions. This rule has been adopted for the Splash as it minimizes any advantage a team would have if they could afford very efficient solar cells. It is also true that with the dimension limits for the boats, 480 is a rather practical limit in terms of size and weight.

How is the Output of the Solar Array Measured?

A calibrated cell is used to measure the incoming solar radiation. Its output is known for one-sun conditions. The ratio of the actual output is divided by the one-sun value. Since the array outputs vary, and cloud conditions can vary greatly, a large calibration box, containing various value rheostats is used to measure the output of the array. Using the ratio obtained by the calibrated cell, the maximum theoretical output of the array is determined. At ambient temperature, this may not exceed 480 watts.

Batteries

There are several reasons that storage batteries are allowed. Should the day be cloudy, it is not very practical to be limited to incoming energy. The batteries are a storage unit for energy, coming from the sun when the boat is not in use. Even on a sunny day, there may be a need for more energy than is currently available directly from the sun. For the Splash, the two hour Endurance event presents the opportunity for 960 watt-hours to be accumulated under ideal conditions. As a result, a nominally equal amount of energy, 1000 watt-hours, may be stored in the batteries. It is understood that one-sun conditions will not exist but neither will it be possible to draw 1000 watt-hours from the batteries in two hours.

Why only Lead-Acid Batteries?

Only electrically rechargeable, lead-acid batteries are allowed due to their relatively low cost, ease of availability, and generally long life. It turns out that the capacity of such batteries is very closely related to their weight. Since weight is easy to measure, especially compared to capacity in watt-hours, weight limits are set for the two types of competitions.

Due to the availability and relatively low cost of commercially manufactured lead-acid batteries, they are used in many marine applications and the Splash. In time, as sophisticated batteries are developed, primarily for electric cars, it is likely that the limitation on lead-acid will be changed.

Why is the energy capacity of the batteries higher for the Sprint?

It is not uncommon for people to assume that electric is synonymous with slow. The Solar Splash has shown that very much to the contrary, from a standing start, an electric drive system is very efficient and pollution free. The limit of 1500 watt-hours is somewhat arbitrary. Since the event is over a 300 meter course, the limit was chosen so as not to limit the power system design. It is probably not possible to use 1500 watt-hours in 300 meters from a battery source.

We could go faster if we were not limited to a "source" voltage of 36 volts.

This is true but safety is of paramount concern to the organizers. In the mid 90's, an international safety standard for DC voltage was established at 52 volts. Since the array must have an output of higher voltage than the battery source, and since battery outputs are nominally in 6 volt increments, the 36 volts represents both a safe and practical value. Deep cycle batteries are usually used as they are not damaged by discharging them to a rather low level of charge.

Recharging of Batteries.

To be consistent with the concept of the Sun being a renewable energy source, only recharging from the solar panels is allowed for the Splash. From the stand point of practicality, the limit of 480 watts means that no one can get an unfair advantage by having extra panels.

Another major consideration is the time table of events. For the Sprints, there are numerous heats but they are spread out over sufficient time that with two sets of batteries, there is very adequate time for recharging. For the Endurance, again, with two sets of batteries and adequate charging time, everyone has the opportunity to start each heat with fully charged batteries.

Inspection of Batteries

In 1997, with teams allowed to choose their own batteries, it was discovered that battery weight is somewhat inconsistent with manufacturers' specs. Starting in 1998, batteries will be weighed and checked against the Manufacturers' specifications which must be submitted with the Technical Reports. No exceptions to the weight limits will be allowed without Manufacturers' specifications.

Other than output, what else should we consider when designing our array?

There are some rather obvious factors such as weight, durability, and configuration. The effect of weight depends greatly on the size and type of hull you are using. Generally, the penalty for a small amount of unnecessary weight is less on water than it is on land. The problem that is more important is the effect of the weight of the panels on the boat's center of gravity. The boat may be very stable without the array but unsatisfactorily unstable with them.

In making these tradeoffs, durability is very important. Panels must be handled quite a bit over the course of the entire event, not to mention testing and transport before arriving on-site. Physical damage to a panel usually results in a loss of output. Finally, although the panels are not on the boat for the Sprint, their configuration can be very important. The panels should generally conform to the hull. If they extend well past the sheer of the boat, they may catch the wind and cause instability on the course. Their configuration should also be considered for launching and retrieval. It is acceptable to add the panels after the boat is launched but then it is extremely important to have an easy, positive way of hooking them up electrically, and securing them in place on the deck.

Suitably large and rugged connectors are far more important than trying to save an ounce or two. They may get wet, and if slippery, may be hard to handle. You should also take into account the use of the array when recharging batteries.

Finally, Maximizers or Peak Power Trackers can be very important. Typically, there may be one of these circuit cards for each panel of the array. The purpose is to maximize the output of any one panel, and then combine these outputs. If this isn't done, the output of the array may be no greater than the output of the weakest component.

Battery Selection Considerations.

There is no doubt that the two primary considerations when selecting batteries are System Voltage and Capacity, but what about other factors? The location of the batteries is one option that allows you a huge amount of leeway in establishing the center of gravity of the boat. It is not just stability, but also the angle of attack or planing once the boat is up to speed.

Safety is of major concern. Many inexpensive batteries cannot take the high rate of discharge required for the Sprint. There are ventilation requirements which must be considered. One location may be ideal but may be directly in front of the skipper, thus requiring a barrier for acid and gas. All of these factors need to be considered in the selection and layout process.

Why are Battery and Motor switches required?

There is a natural tendency to assume that a Motor shut-off switch would be adequate. In most cases, it is not. Depending on the circuit, the output of the array may power the boat even if the Batteries are turned off. In recent years, it has become more common to use a relay, rather than long runs of large diameter cable, to accomplish the switching and thus reduce the size and weight of switches and cables. It is still vitally important to size cables correctly for anticipated current.

Is there a need for a great deal of instrumentation?

It depends on many things. Just like an airplane, the boat can be operated with a bare minimum of instruments or use a telemetry system which is quite sophisticated. Much depends on your operating strategy. You must consider the tradeoffs in reliability, weight and complexity.

What is your Energy Management Strategy?

For the Sprint, it is most likely "give it all it can take!" but the Endurance is another matter. Generally, the circuit philosophy is to have all of the output from the array go to the motor and then add power from the batteries as conditions dictate. Key questions are how much and when. Certainly, knowing the discharge characteristics of the battery pack is critical but what about the weather? A good forecast of solar energy is very important. Where do you stand with respect to your nearest competitor? How much time is left in the Heat? Since distance is measured to the nearest 1/4 lap, where do you expect to finish? These are just some things worthy of consideration.

It also may be helpful to have a cloud cover forecast for re-charging in the Paddock area. How full are your various sets of batteries? Do you have a "top off" strategy?

Can you monitor key systems to know that they are performing properly? If the output of part of your system is not up to expected, do you have a strategy team to deal with the questions which arise?

Summary

The purpose of this section is not to tell you what to do nor how to do it. The purpose is to give you a reference on some of the philosophy behind the rules and, to perhaps give you a source from which you can develop a check list of items you need to research or cover in your preparations for the event. It is very important to remember that the energy system is probably not much better than the weakest link.

Remember, if you have questions, do not hesitate to contact Headquarters at hq@solarsplash.com.