So­lar panels

Emhrys Bar­rell looks at the lat­est tech­nol­ogy in so­lar panels, which are now pro­duc­ing mean­ing­ful amounts of power, at sen­si­ble prices

Practical Boat Owner - - Contents -

Which tech­nol­ogy will work best for your boat?

So­lar tech­nol­ogy has been im­prov­ing steadily over re­cent years, to the point where it can produce real, use­ful power on even the small­est boat, able to run a fridge, LED lights and in­stru­ments when you are away from shore power, or main­tain the charge in your bat­ter­ies when you are away from the boat. And the huge in­crease in so­lar gen­er­a­tion across the world had brought prices down to rea­son­able lev­els. We will look now at the changes in the tech­nol­ogy.

Charge con­trollers

The out­put of a so­lar panel is mea­sured in watts, be­ing the prod­uct of the volt­age times the cur­rent. The volt­age out­put of all so­lar panels is de­pen­dent the amount of sun­light fall­ing on them, and the load they are con­nected to. If it is un­re­stricted, the volt­age of a nom­i­nal 12V panel can rise as high as 17-21V, de­pend­ing on the con­di­tions. The rated out­put will be de­liv­ered at a lower volt­age than this max­i­mum, but will still be 16-18V. How­ever, this fig­ure is higher than the safe charg­ing volt­age of a 12V lead acid bat­tery.

Above 14.4V, open bat­ter­ies will give off hy­dro­gen and oxy­gen gas, while sealed gel or AGM bat­ter­ies can be per­ma­nently dam­aged. If the out­put of the panel is small com­pared to the ca­pac­ity of the bat­tery bank, the in­ter­nal re­sis­tance of the bat­tery will keep the volt­age down, but for most panels over 20W out­put you will need some form of charge con­troller.

The early con­trollers used pulse width mod­u­la­tion (PWM) to keep the bat­tery volt­age un­der the gassing point. This was sim­ple and hence cheap tech­nol­ogy, but the down­side was that as the out­put cur­rent re­mained un­changed, you were ex­tract­ing less than its max­i­mum power from the panel. For ex­am­ple, a nom­i­nal 100W panel, with a max­i­mum power volt­age of 17.0V, will de­liver a cur­rent of 100 ÷ 17.0 = 5.9A. If this is reg­u­lated down to a charg­ing volt­age of 13.0V, you will get an out­put of 13.0 x 5.9 = 77W: a 23% re­duc­tion from the rated fig­ure.

This isn’t a prob­lem if you’re just us­ing the panel to main­tain the charge in your bat­ter­ies and counter off-load self­dis­charge. How­ever, if you want the panel to de­liver the max­i­mum use­ful power, a bet­ter con­troller is needed.

These are now avail­able in the form of MPPT, max­i­mum power point track­ing units. You can find de­tailed ex­pla­na­tions of how these work on the in­ter­net, but briefly they act as a DC:DC con­verter, tak­ing the in­com­ing volts and amps from the panel and con­vert­ing them to volts that the bat­tery re­quires, but main­tain­ing the over­all power that the panel is de­liv­er­ing in watts (less a small con­ver­sion loss). Hence it de­liv­ers more cur­rent to the bat­tery than the panel is de­liv­er­ing, and over­all de­liv­ers close to 100% of the out­put power of the panel. This gives them a sig­nif­i­cant ef­fi­ciency gain over PWM con­trollers, up to 30%.

In the ex­am­ple above, the charg­ing cur­rent will be 100 ÷ 13.0 = 7.7A: a 30% im­prove­ment.

The max­i­mum power point is the volt­age at which the power from the panel in watts is great­est. This varies with load and tem­per­a­ture, and the MPPT con­troller is con­tin­u­ously mon­i­tor­ing this point,

hence the name. Look­ing at the maths, it can be seen that the ef­fi­ciency gain of MPPT over MWM is pro­por­tional to the dif­fer­ence in volt­age of the bat­tery and the panel. The larger this dif­fer­ence, the higher the dif­fer­ence in out­put of MPPT ver­sus MWM. This is af­fected by sev­eral fac­tors.

The first is bat­tery volt­age. Charg­ing volt­age in the bulk phase, up to 80% ca­pac­ity, is nor­mally 1.0V above the bat­tery volt­age. So a flat bat­tery, with a volt­age of 11.7V, will have a charg­ing volt­age of 12.7V. In our ex­am­ple above, this will mean the MPPT con­troller will de­liver 100 ÷ 12.7 = 7.9A.

As the amount of charge of the bat­tery in­creases, its volt­age goes up – up to 12.5V in the bulk phase – and the charge volt­age up to 13.5V. At this point the cur­rent will have re­duced to 7.4A, so the ef­fi­ciency gain over PWM will have re­duced slightly.

The second fac­tor is the tem­per­a­ture of the cells in the panel. As the tem­per­a­ture of a cell in­creases, the volt­age that the max­i­mum power is de­liv­ered at drops, and hence the ad­van­tage of MPPT over MWM re­duces. This re­duc­tion be­comes most pro­nounced as the cell tem­per­a­ture in­creases above 40-50°C. Cell tem­per­a­ture will be higher than am­bi­ent, by an amount that varies be­tween panels mounted on a flat sur­face, and those stand­ing in free air, but will be around 10°C. There­fore the PMMT ben­e­fit drop-off starts to be­come sig­nif­i­cant in am­bi­ent tem­per­a­tures above 35-40°C, so only of limited over­all ef­fect in our tem­per­ate cli­mates, but more sig­nif­i­cant as you move to­wards hot­ter climes.

The MPPT con­troller ad­van­tage also in­creases as the amount of sun­light fall­ing on the panels is re­duced by shad­ing, par­tial cloud or low an­gles of sun, at the be­gin­ning and end of the day or in win­ter. This is a def­i­nite ben­e­fit on our boats, where par­tial shad­ing from masts and rig­ging can be

a prob­lem.

Re­duc­tion in panel out­put

When you are mak­ing cal­cu­la­tions as to the likely out­put you will get from a given so­lar panel, you have to bear in mind that the rated out­put, or the head­line fig­ure that it will be de­scribed and sold un­der, is the max­i­mum that it will ever produce. This will only oc­cur if the sun­light is fall­ing di­rectly on it, at 90°, in clear air.

For a panel laid flat on the ground or deck, this will only oc­cur at mid­day, be­tween the tropics, and at cer­tain times of year. In our lat­i­tudes, the fig­ure will al­ways be re­duced be­cause the sun is never di­rectly over­head, al­ways be­ing an­gled to­wards the south, by an amount that varies from sum­mer to win­ter due to the tilt of the earth on its axis and its or­bit round the sun. It is also re­duced through the day, as the sun rises and sets, go­ing from an an­gle of zero at dawn and dusk, and max­i­mum at noon.

The amount of light that gets through is also af­fected by the amount of at­mos­phere it trav­els through. This is min­i­mum when it is over­head, but as the an­gle dips to the hori­zon, the amount of air it has to travel through in­creases, and the avail­able light is dis­persed. You can ob­serve this ef­fect of the scat­ter­ing of the light when the sun or moon dip to the hori­zon, and their ap­par­ent di­am­e­ters in­crease.

The ef­fect of the re­duc­tion due to the an­gle of in­ci­dence can be coun­ter­acted by tilt­ing the panels to­wards the sun, which is why all so­lar farms have their panels an­gled to the south – but this is rarely prac­ti­cal on our boats. Some craft on rivers and canals have taken ad­van­tage of a fur­ther ef­fect by mount­ing some of their panels ver­ti­cally on the cabin sides, so they get not just the light di­rect from the sun, but the re­flected light from the wa­ter sur­face – but again, this is a special sit­u­a­tion.

The amount of light fall­ing on the panels will also ob­vi­ously be af­fected by cloud, with even wispy high cloud hav­ing some ef­fect, right down to the dark­est storm clouds bring­ing the cur­rent to near zero. The over­all re­sult of these re­duc­tion is that for re­al­is­tic cal­cu­la­tions, you should as­sume a max­i­mum of

3-5 hours sun­light a day in sum­mer, with around 80-90% of the rated out­put at mid­sum­mer, noon, fall­ing away ei­ther side of this and to­wards win­ter.

Panel de­sign

Three types of so­lar panel are com­monly avail­able: rigid, semi-flex­i­ble, and flex­i­ble. Their de­sign and man­u­fac­ture have changed rapidly over re­cent years, driven by the in­crease in use world­wide for re­new­able power gen­er­a­tion.

Rigid panels have a frame, which means they can be an­gled to­wards the sun, but are less ro­bust and can­not be bent or walked upon. Semi-flex­i­ble panels can be bent to the curve of a deck, and can be walked on in soft shoes, but they must be sup­ported, and can­not be re­peat­edly bent back­wards and for­wards or the con­tacts be­tween the cells can break. Fully flex­i­ble panels can be curved to any shape and even rolled up, but their out­put is lower than the other types.

The panels are made up of sep­a­rate cells of sil­i­con, con­nected in se­ries and par­al­lel, and these can be ei­ther monocrys­talline, poly­crys­talline or amor­phous. Amor­phous cells are com­monly used in low-power ap­pli­ca­tions, such as cal­cu­la­tors, toys and small, low-out­put panels. Crys­talline cells are made from molten sil­i­con that is poured into a mould then sliced into thin cells, mak­ing them very ef­fi­cient. Monocrys­talline cells tend to have greater out­puts, es­pe­cially at higher tem­per­a­tures, but the two are be­com­ing sim­i­lar in per­for­mance.

The cells have to be elec­tri­cally con­nected by metal­lic strips, and on the ear­li­est de­signs, these strips were printed on the front and back of the panels – pos­i­tive on one side and neg­a­tive on the other. How­ever, the strips on the front face shielded the cells from the sun­light to a de­gree, and the lat­est de­signs now use back-con­tacts only, which im­proves the per­for­mance of a panel from 16-18% of the light fall­ing on it to 20-25% ef­fi­ciency.

The sur­face of the panels also has an ef­fect on per­for­mance. Rigid panels will typ­i­cally have glass pro­tect­ing the cells, but this can re­flect some of the sun­light fall­ing on them, par­tic­u­larly at low an­gles of in­ci­dence. Var­i­ous al­ter­na­tive coat­ings are used on semi-flex­i­ble panels, which pro­tect the cells when they are walked on, but can also ac­tu­ally in­crease out­put at low an­gles of in­ci­dence by act­ing as tiny lenses and di­rect­ing the sun­light onto the cells. The cells are all linked to a fi­nal con­nec­tion block, with pos­i­tive and neg­a­tive out­puts. If this block is mounted on the face of the panel, it can trip you up when walk­ing across it. The al­ter­na­tive is to mount

Semi-flex­i­ble panels can be bent to the curve of a deck, and even walked on

the block on the back face, giv­ing a smooth sur­face, but if the panel is to be mounted flat on the deck, this block will have to be re­cessed into it.


The panels will have to be mounted on the deck. Rigid framed types will need brack­ets, which means they can be an­gled to­wards the sun. This is use­ful on nar­row­boats, for in­stance, which can spend some time moored in one location. This will also al­low air to flow be­hind the panels, in­creas­ing their out­put at higher am­bi­ent tem­per­a­tures. How­ever, they are not prac­ti­cal on sailing boats. For these you will have to use semi-flex­i­ble panels, fixed to the deck. This can ei­ther be done with screws and bolts, or they can be stuck down. Some panels come with peel-off sticky back­ing to al­low you to do this quickly and sim­ply. Oth­er­wise you will have to use a thin con­tact glue or mas­tic.

As we have said, the semi-flex­i­ble type should not be re­peat­edly re­moved as this can break the del­i­cate in­ter-cell con­tacts.

Con­nect­ing ca­bles should be as large a di­am­e­ter as pos­si­ble, to re­duce the volt­age losses, and 6mm2 wire is gen­er­ally ac­cepted as the min­i­mum for medi­umpower in­stal­la­tions. The ac­tual joints be­tween cable and con­nec­tors should also have a large con­tact area and be wa­ter­proof, and it is best to use the con­nec­tors pro­vided by the panel sup­pli­ers. These will come marked pos­i­tive or neg­a­tive at the ends of each of the link wires, but as we moved through the sys­tem from panel to con­troller and then to the bat­tery, we found that the po­lar­ity of each cable ap­peared to change.

To avoid con­fu­sion, work on the prin­ci­ple that the out­put from the panel is king, and mark all cable ends along from the pos­i­tive out­put with red in­su­lat­ing tape, and from the neg­a­tive with blue tape. This will avoid any pos­si­bil­ity of cross-con­nec­tion. While most con­trollers will ac­cept re­verse po­lar­ity, it is best not to put this to the test. When wiring up the sys­tem, al­ways con­nect the bat­tery to the con­troller first.

If pos­si­ble, avoid mount­ing panels where per­ma­nent shad­ows from the mast or rig­ging will fall on them. This used to com­pletely cut off the power out­put, but mod­ern tech­nol­ogy has con­sid­er­ably re­duced this ef­fect. How­ever, it should still be avoided if you can. Sim­i­larly, do not mount them be­hind a win­dow as this will also cut the cur­rent out­put con­sid­er­ably.

The test

We tested a se­lec­tion of panels of dif­fer­ent types and out­puts, from 2.4W up to 150W. This was not in­tended as a test of one make against another, but to in­ves­ti­gate the dif­fer­ent de­signs, sizes and equip­ment avail­able, and the rel­a­tive per­for­mances of the two types of con­troller. We also in­cluded panels that would charge your phone, tablet or lap­top, this be­ing the most ba­sic power re­quire­ment most of us would have.

We tested the out­put of each panel into two iden­ti­cal bat­ter­ies – one 10% full, with a no-load volt­age of 11.7V, and one 75% full, with a no-load volt­age of 12.5V.

We tested the out­put at mid­day, at the end of June, the time of max­i­mum out­put of the sun in the north­ern hemi­sphere, and then at 4pm to gauge the re­duc­tion in power. We also tried a day when the sun was partly ob­scured by high cloud. We laid the panels flat, on a sheet of ply­wood, to sim­u­late the typ­i­cal in­stal­la­tion on a deck, so the out­put we recorded was al­ways go­ing to be slightly down on the max­i­mum pos­si­ble if we had an­gled the panels at 90° to the sun, as even in mid­sum­mer the sun is never ex­actly over­head at our lat­i­tude.

We con­nected each through a PWM con­troller, and then an MPPT con­troller. We mea­sured out­put from the panel to each con­troller, in volts and amps, then from the con­troller to the bat­tery. The MPPT con­troller had a dig­i­tal dis­play of in­put and out­put pa­ram­e­ters, but we dou­ble-checked this with our own am­me­ter and volt­meter.

We mea­sured the over­all di­men­sions of each panel, then the size and area of the cell ar­ray to al­low us to cal­cu­late out­put per square me­tre.

The face of a back-and-front-con­tact panel, showing the con­nect­ing strips printed on the sur­face, and the con­nec­tion block

Our test rig, with so­lar panel, con­troller, bat­tery and clamp am­me­ter

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