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PV systems, Electrical lighting distribution: what are pros and cons of a low voltage DC network vs 230VAC network

I am involved in PV projects, mostly electrification of schools, medical centers etc; I used to work with low voltage DC distribution but with the availability of more powerful PV panels rating 24VDC, I presume that it is preferable to go for a 230VAC system, also for lighting purposes.

Typical cases I am working on are e.g. the lighting (for evening courses) of two classrooms, each 8mx4m, together with lighting for the manager’s office (4mx4m) as well as exterior lighting. On another occasion I look into the lighting of a medical centre, encompassing several rooms in one building with a total ground surface of approx. 12mx6m.

What I want to know is if there is any preference, eventually depending on local circumstances, to be given to a 12VDC or a 230VAC distribution for only lighting.

This relates to a number of countries (as a member of Engineers without Borders/Belgium) that are interested in energy and water supply with current projects in countries including Togo, Cameroon, Mali, Burkina Faso, Uganda, and India.

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With low voltage DC systems you are fairly limited in how far you can run the wiring before voltage drop issues begin to cause problems. BUT low voltage DC systems are much safer since under ~50 volt many electrical codes consider to be safety low voltages, and generally not an electrocution risk. (Yes, if both conductors were inside human tissue there is a higher risk) Also many low voltage DC systems are run ungrounded (unearthed) where you would have to make contact with both conductors for any current to flow. See - http://www.filebuzz.com/findsoftware/...

The 230 VAC system has a much longer distance it can be run for the same wire size before voltage drop issues become a problem, but it also has shock risks with contact with the hot conductor and ground (earth). As for positive points, almost any normal household load can be handled with a properly designed 230 VAC system.

In general LED type lighting is available for both systems, but larger loads above say ~100 watts are fairly rare for 12 VDC systems. So 12 volt systems can handle lights, TV's, radios, small fans, and some very small electrical refrigerators and such, but aren't practical for cooking appliances, pressing irons or other large wattage loads without specially designed systems with very large wire sizes.

Larry Bentley EWB-USA Energy Content Committee

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As above

transmission losses for DC systems are greater, and you will be limited to the amount of stuff that you can power - but if it really only for lighting, and people wont want to add extra stuff once they have the power (which is pretty common), DC will be fine - over short distances.

One disadvantage of AC is that if your PV is 24 VDC, then there is the cost of the inverter to give you 230 VAC.

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Just to feed into the points and advises that have been made on the pros and cons of DC as opposed to conventional AC. Using PV panels to supply the type of building you have mentioned can be problematic. Using DC you can limit power theft and discourage unapproved cable connections which will drain batteries and make the system less sustainable/costly and higher servicing costs. However if you go with 36-48 volt DC then you can keep cable sizing for the type of room sizes you have highlighted down to 4-6mm which can be easily purchased in the countries you have listed and at a normal price.

When installing DC lighting systems you should seriously consider no cutting back on installation costs, 1-2 lights per light switch maximum, as you want to be able to achieve maximum energy efficiency and with DC circuits at 36-48 volts DC circuit breaker current will be very high so 2-3 lights per breaker. For each room having independent systems is important so that if a room is over used this does not lead to the draining of an entire battery bank. Isolate the potential to bring the whole system down by breaking the system into 2-4 smaller systems. 2 class rooms plus an office and external lighting ideally should be 4 separated systems.

If you decide to use the PV, DC to AC (inverter-charger systems) then you need to consider the losses in energy (cable losses and DC-AC conversion losses) and oversize the system accordingly. Using any existing wiring systems will increase losses and battery drainage issues so best to install new wiring to assure the system is efficient. Keeping circuits and system small is still a good design, when using this system so that sustainability and rehabilitating failing loading designs is easy.

Battery drainage is vital with both types of systems so getting a company to install a system which is programmed to disconnect and protect batteries at 50-40% drainage is vital. Depending on the type of batteries this will assure that lifespan will be 3-5 years as opposed to 12-18 months.

The AC system will be the easy system to buy materials locally, for example lights, but the DC system will require ordering materials and having spares plus correct training and M&E but have better sustainable characteristics.

The option of having a DC lighting system with an AC inverter-charging setup should also be considered as buildings often serve a multiple of tasks and lighting may be required for literacy requirements.

Best of luck

Paul

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Asked:
2014-11-14 05:01:14 -0500
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Last updated:
Nov 16 '14