What are you trying to chill? Cool drinks are good for morale, but hardly essential for life. Chilling food can be crucial for hygiene. Keeping vaccines cool is a life saver. Knowing the value of y...(read full answer)
What are you trying to chill? Cool drinks are good for morale, but hardly essential for life. Chilling food can be crucial for hygiene. Keeping vaccines cool is a life saver. Knowing the value of your payload helps prioritise and weigh up how much effort and expense to allocate.
Non-critical items can be partially cooled using an evaporative cooler. This uses a porous shell from which water evaporates, drawing heat from the inside. They work best in conditions of low humidity. Air movement also helps. A simple version is a large earthenware pot with a lid, sitting in a bowl of water or saturated sand. Another approach is to build a box with mesh walls filled with charcoal and with a leaky water container or pipe dripping into the top of the walls. At its most basic, you can use a damp cloth over a metal box. There are plans and an explanation of a charcoal cooler here: http://www.appropedia.org/Charcoal_Co...
Thermo-electric or Peltier effect coolers pass electric current through two different materials causing heat to flow from one junction to the other. Cheap examples are often available as small cool boxes for use in vehicles as a picnic cooler. They are inefficient compared to other powered systems.
Absorption fridges use heat from burning LPG or kerosene to drive an ammonia-based cooling circuit. They are fairly cheap but require a fuel supply. They are generally simple and reliable as there are no moving parts, but have poor temperature control and can freeze contents. Historically, they were used for vaccine and medicines but are no longer considered best practice. The multi-fuel fridges (230Vac, 12Vdc, LPG) found in camper vans and caravans use this technology.
Compression refrigeration cycle is used in conventional mains powered fridges. Beyond basic domestic versions there are specialised medical fridges, direct solar and 12 Vdc fridges. If you are using an off-grid power supply there are several options:
Use batteries, an inverter and an AC fridge. The fridge is relatively cheap but you will need the other components. If you are using a battery-charger-inverter system to supplement a generator or unreliable mains, this is an option, but bear in mind that a fridge is relatively energy hungry compared to a laptop or lights.
Use a specialised DC fridge which cost more but are more efficient. They use a specialised compressor and controller and will run off a 12Vdc battery system. Typically they are better insulated to reduce energy demand.
Solar direct drive fridges have been developed for the medical market that run direct from PV modules with no need for batteries, which are often the first component to fail in a DC fridge system. To keep the fridge cold overnight, they store cool in ice or phase change materials. There are also versions that work with intermittent mains supplies.
WHO have a performance standard (PQS) to which all fridges used in UNICEF supported cold chains should conform. This stipulate the internal temperature variation (between 2 and 8 C), holdover (how long safe temperatures are maintained without power) as well as aspects like durability and labelling. More details are here http://apps.who.int/immunization_stan...
Cool boxes: in a situation like a large refugee camp with centralised medical facilities and distributed health outposts, it may be worth considering using ice packs or chilled water packs in insulated boxes to keep vaccines and medicines cool away from the main facility. The packs will need replacing every day or so. Take care not to freeze the vaccines – the ice packs should be ‘conditioned’ (allowed to start melting so they are at 0C) before use. Alternatively, chilled water packs store less cool but ensure the
The simple, and obvious, answer is ‘by pointing it at the sun’! However, this can be more involved than it first appears. The sun tracks across the sky from East to West and at its zenith (highest ...(read full answer)
The simple, and obvious, answer is ‘by pointing it at the sun’! However, this can be more involved than it first appears. The sun tracks across the sky from East to West and at its zenith (highest point, around midday)) will generally be in the direction of the equator – South in the northern hemisphere, and North in the southern hemisphere. Near the equator at midsummer, it can overtop to the opposite pole. The vertical angle of the sun (azimuth) varies through the seasons, highest at midsummer and lowest at midwinter. The midday azimuth at the equinox (midpoint between midsummer and midwinter) is the same as the angle of latitude for the location.
A photovoltaic (PV) module (‘solar panel’) will receive the most energy when it is pointing directly at the sun. Ideally, the module would track the sun east to west during the day, and tilt higher or lower during the seasons. Many plants do this, sunflowers being a great example. Tracking systems are available but these are generally expensive and inherently less reliable than a fixed mounting. It is quite simple to arrange a frame that can be raised or lowered every few months to match the season, but this adds the potential for human failure (forgetting to change it from winter to summer angle) and reduces one of PV’s best points that it is virtually ‘fit and forget’. The cost of PV is now so low that it is usually most cost effective to fit a slightly larger panel and orientate it for the best average performance for the year. The simplest approach is to mount the panel pointing towards the equator tilted at an angle from the horizontal equal to the latitude of the site. So, in Zimbabwe, this would be facing North at an angle of about 20 degrees, and in Pakistan it would be South at about 30 degrees.
If you are mounting the module on an existing structure such as a roof, it may not be pointing the right way. It is possible to set up a frame to adjust for this, but this can be quite fiddly. In general, if the main direction is within 45 degrees of the equator direction the loss of output is only a few percent. There are a couple of other considerations for the tilt. Output from the module will reduce significantly if it gets dirty. Where rain is very seasonal, it is important to clean the panel during the dry season (when there is also typically more dust). However, it is best practice to use at least 10 degrees of tilt to help self-cleaning during rain, even at the equator (with latitude zero).
The second consideration for tilt angle is how the load varies with season. In high latitudes, there is a significantly shorter day length in the winter, when a system intended primarily for lighting will have a much higher load, whilst at the same time receive a lot less solar energy as the sun is not in the sky for long. In this case, the module (or array – a collection of modules) should be mounted at an angle to horizontal 10 to 20 degrees steeper than the location’s latitude to maximise the solar energy when the sun is low in the sky. In the UK, you will often see this with solar powered streetlights with modules at angles of 60 degrees or more. On the other hand, a load that increases in the summer, such as refrigeration, would justify using a flatter tilt angle than latitude to get more energy from the higher summer sun.
So far, so good – point it at the sun and keep it clean.
However, there is a potentially more significant cause of lost energy than dust or slightly wonky orientation – shading. Because of the way silicon wafers are connected together to make a PV module, it is possible for a shadow covering 10 – 20% of the module to reduce its output by 50 - 80%. It’s not always this bad and depends a lot on the configuration of the module and the type of load e.g. battery or direct inverter. But it is always worth trying to site the module away from potential shadows. Typical culprits are buildings and trees, but even quite slender objects like poles, wire fences and aerials can cause significant losses.
The quick and dirty way to do this is to look towards the equator (a compass helps), then imagine the suns path at the equinox by looking up at the angle of latitude (don’t look directly at the sun, though). Make a rough trace to the east and west to estimate the sun’s path. Waving your arm about can help! Tilt up and down about 15 degrees for winter and summer paths. Most solar energy is received between 9am and 3pm. At the equinox, the sun is in the sky for 12 hours so if it is highest at midday, at 9am it will be halfway up and 3pm halfway down. It is hard to avoid all shading early and late in the day, but if you can get a clear sky between these hours you will harvest the bulk of the available energy.
Try to move the mounting away from shading (but not too far from the load as longer cables lose more energy in transmission). Getting higher is often the best policy, but remember you need to be able to clean the panel somehow. Maybe you can remove the shading object. Also remember that trees and bushes grow upwards and may cause shading in a year or two.
Tracing an estimated sun path by waving your arm is very rough and ready. There are physical tools to measure the path using clear plastic screens traced with sun paths for different seasons and latitudes, but these are not that easy to find or use. A great alternative is to use a solar tracking app for a smartphone. It is possible to use a star chart app like SkyMap to trace the sun, but there are several excellent apps designed specifically for solar installers that display the sun’s path at different seasons on the camera screen as you point your phone at the sky. There are several free versions available, but the ‘professional’ ones are often only a few dollars. Sun Surveyor for Android and Sun Seeker for IOS are good examples but there are others available. There are also more simple apps that turn your phone into a clinometer and tell you when you when it is in the best orientation for the PV module e.g. SolarTilt. Another useful (Android) app is Scan the Sun where you trace the horizon and the app calculates the potential output for different months of the year. If you search the app stores for Android or IOS there are quite a selection. Many are geared towards grid-connected systems but the shading and orientation tools work whatever the application.
In summary, for general applications
- Point the module towards the equator
- Tilt it at the angle from horizontal equal to the site’s latitude, but not less than 10 degrees
- Position the panel to avoid shading between 9am and 3pm, thoughout the year
- Remember that trees and plants will keep growing!
- Smartphone apps can be simple tool for assessing shade and getting orientation right.
I work in an agricultural training centre in a small town. We have a mains electricity supply but we get power cuts regularly. In the rainy season, the power is only available 4 to 8 hours a day an...(read full answer)
I work in an agricultural training centre in a small town. We have a mains electricity supply but we get power cuts regularly. In the rainy season, the power is only available 4 to 8 hours a day and sometimes goes off for several days at a time. We use electricity for water pumping, lights, fridges for the canteen, computers, TV, kettles and power tools, among other loads.
A common approach is to use a diesel (or for small loads, petrol) generator (‘genset’), but this is expensive and noisy to run. An alternative solution is to use a battery, charger and inverter. When mains is available, the charger fills the battery. When there is a power cut, a changeover switch connects (some of) the loads to an inverter which converts the 12 or 24 V DC (direct current) of the battery to 230 (or 110) V AC (alternating current) like that provided by the grid.
A similar approach can also be used with an off-grid system running on a generator, to reduce the run time to periods of a few hours for high loads such as cookers, washing machines, heavy power tools, air conditioning and pumps. The inverter and batteries can then supply smaller loads such as lighting, computers and TV for the rest of the time, removing the need to run a large genset just to keep a few lights on.
Inverter rating Add up the power rating of all the loads that you think may be required to run at the same time. The inverter rating (in watts or kilowatts) needs to be higher than this value. If you have any large motors, e.g. pump, or compressors e.g. fridge, freezer, these have high starting currents, 3 to 6 times their running value. Good inverters should be able to supply brief peaks above their rated output. Check the specification.
Useful inverter features
Wave form: inverters convert direct current (fixed voltage) to alternating current (voltage varying positive to negative in a sine wave). The most basic devices do this by creating a square wave. A ‘modified sine wave’ is effectively a stepped square wave. The best inverters produce a pure sine wave (or a good approximation). These are the best to use as they work better with inductive loads like motors and fluorescent lights and produce less interference.
Low voltage disconnection: helps to protect the battery from over-discharge by switching off when the battery voltage is too low. Over-current protection: most inverters are protected from connecting the battery the wrong way round, although this might blow a fuse. Similarly, they should disconnect if the load is too large or has a fault (short circuit).
Sleep mode: better inverters have the ability to switch off when there is no load, sending an occasional pulse to detect if something has been connected, in which case the inverter fires up and provides 230 Vac (or 110 V). This reduces the standing losses when idle to a few watts.
Inverter-chargers: some inverters can also ‘work backwards’ as battery chargers and include a transfer switch to automatically change supply from mains to inverter. They may also provide a start signal to an automatic generator. A variant of this type is known as an uninterruptable power supply (UPS) and can switch from mains to battery power fast enough for loads such as computers not to notice the change in supply.
You will need deep-cycle batteries, most likely lead-acid. Don’t use vehicle batteries (SLI, starting, lights & ignition) as they won’t last long. More details on how to look after and size the battery bank are provided in the KnowledgePoint post “How do I size a battery bank?”.
The charger needs to refill the battery within the time that power is available from the grid or generator. Once you have worked out the daily battery load in amp-hours (see “How do I size a battery bank”), divide this by the available charging time to get the charging current. Don’t try to charge the battery too fast. The maximum charge current should not exceed C/5 (where C is battery capacity in Ah) and C/10 is better. Sealed batteries should be treated more gently as fast charging can cause gassing and they cannot be topped up with water like a vented battery. Sealed batteries should be charged at a slightly lower voltage than vented. Good chargers will have a battery type setting.
Any system involving 230 V supplies should be installed by a competent person such as a qualified electrician. Be sure to connect the earth terminal on the inverter to the system earth. Any circuits feeding sockets should be protected by an earth leakage trip (residual current device, RCD, RCBO).
Unless it is built into your inverter, you will need a transfer switch. This should be 2 pole, 3 position, or, in other words, switch both live and neutral wires between mains/generator and inverter with an intermediate Off position to ensure that you never end up with live and neutral connected to different sources. Typically, it is a rotary switch.
The inverter system should only supply the critical loads – lights, computers, communications, and possibly refrigeration. These should be on a separate circuit that can be supplied by either mains or inverter, whilst the heavy loads can only run on the mains.
The starting point is to assess how much energy in watt-hours (Wh) is needed. Laptop batteries are typically 11 – 15V voltage with a capacity of 2,500 mAh (= 2.5 Ah) to 7,000 mAh.
Voltage (volts,...(read full answer)
The starting point is to assess how much energy in watt-hours (Wh) is needed. Laptop batteries are typically 11 – 15V voltage with a capacity of 2,500 mAh (= 2.5 Ah) to 7,000 mAh.
Voltage (volts, V) x Current (amps, A) = Power (watts, W).
Power (watts, W) x Time (hours, h) = Energy (watt-hours, Wh).
Capacity (Ah, = 1,000 mAh) x battery voltage (V) = Energy storage (Wh).
Laptop batteries are generally between 40 and 80 Wh. It’s useful to convert from the typical laptop battery specification of milliamp-hours because the voltage that you generate may not be the same as that used by the laptop. Defining the energy required is the best way to compare different sources.
The next question is how often you need the battery recharging? A fully charged laptop will typically run for 3 to 6 hours depending on the model, activities, power settings, and age of the battery. Is this enough for one day, or several? This will tell you the minimum amount of energy you need to generate per day.
How will you charge the laptop? Laptop power supplies convert from 230 Vac (or 110 Vac) to 15 to 20 Vdc. If you have a 230 Vac source – an engine or hydro generator – you can use the standard laptop power supply. Whilst it might sound feasible to connect a DC (direct current) source, such as a solar PV panel, direct into the laptop charging socket, you will probably damage the charge regulator in the laptop. A practical solution is to use an intermediary 12V battery and either a small inverter to convert to 230 Vac or a 12 Vdc car adaptor. Inverters are easier to find (or replace) locally but may result in other loads being connected.
How to generate the energy? Unconventional sources like piezoelectric (shoes, pavements) or thermoelectric (stove or fire-charger) sound like an interesting research project. Small stove chargers are available but generally output to a USB socket which at 5.2 Vdc is not enough for a laptop. Although technically less exciting, the best approach is to use an easily available, well proven source. Typical examples would be solar PV, engine generator, wind turbine, hydro generator or, maybe, pedal generator.
For the small amount of energy required, an engine generator is overkill, and requires regular and costly fossil fuel to operate. Nepal has for many years been a world leader in the use of micro hydro for village power supplies. There may well be one available locally, in which case the laptops could be charged directly, or possibly via a portable battery (with inverter) that can be taken for charging to the hydro system once a week. Hydro systems are quite expensive to install, so would not be cost effective just to run a few laptops alone. As a village electrification supply it’s a different story.
Wind turbines are well proven and can be made locally – see Hugh Piggott’s designs at scoraigwind.co.uk which have been adapted and built in small workshops all over the world. However, windpower is very site specific and needs a well sited tall tower (see KnowledgePoint Q&A “What makes a good windpower site”).
Pedal generators have been trialled in Tanzania and proved popular at UK festivals and there are various designs available on the web. One person could generate 50 – 100 W for a sustained period so an hour’s pedalling could fully charge the laptop.
The simplest solution is a small solar PV panel with a charge controller and a small 12 V deep-cycle lead-acid battery. 10 to 40 Ah capacity (120 – 600 Wh) would work for a couple of laptops. Nepal mean daily solar irradiance is between 4 and 6 kWh/m2. This can also be expressed as peak hours which sums the variable irradiance from dawn to dusk and converts it to hours at 1 kW/m2, i.e. the same figure as for kWh/m2. A 50 W PV module (solar panel) will provide 50 times the peak hour rating – 200 to 300 Wh in this case so enough to provide some extra to cover cloudy days (but you need sufficient battery capacity to store the extra).
Costs vary locally, but a rough budget might be
50 W PV module £60
50 Ah sealed deep-cycle battery £70
5 A charge controller £20
100 W inverter (basic) £60
Simple mounting, cable, fittings£40
Total ~ £250
What do you want to achieve? If you are looking for an interesting R&D project, by all means explore unconventional energy sources. If you want a simple, cost effective solution that can be set up and run with minimum skills in most off-grid locations, a small solar PV system is the most likely choice. There are still technical challenges to consider. Can you build the components into a robust and portable format? What happens when the battery needs replacing. Do you add in USB sockets to charge phones, and if so, how do you allow for the additional load? What about a light? It’s always time well spent talking to potential users to find out what they need electricity for and what they currently use.
A: If a small solar photovoltaic (PV) system is to stay in one place for more than a day or two, the modules (panels) need securely mounting to protect them against wind and theft. There are 3 main...(read full answer)
A: If a small solar photovoltaic (PV) system is to stay in one place for more than a day or two, the modules (panels) need securely mounting to protect them against wind and theft. There are 3 main approaches, the choice of which depends on scale, budget, available skills and materials, and local conditions. The main requirements are for the array to be secure, close to the load, pointing the right way and not shaded between 9am and 3pm. It also helps if the array can be safely cleaned by the users.
Roof mounting This is usually the simplest approach, assuming the load is in a building. The roof offers some height to avoid shade and interference. Preferably find a roof pitch pointing north (southern hemisphere) or south (northern hemisphere). See Knowledge Point Q&A on ‘how to set up PV for best output’ for more advice on orientation and shading. Working on a roof is hazardous. Check the condition and layout of the roof structure before climbing on it. Use boards to spread the load and consider how to get the modules on the roof safely. Modules are best attached to rails – ideally aluminium or galvanised / painted steel, though treated timber will work – using clamps on the frame. The rails can then be attached to the roof substructure – purlins or rafters, not the roofing sheet alone - using long coach screws, J-bolts or stand-off bolts (wood thread one end, machine thread the other, with a roof washer in the middle). Make holes on the ‘hills’ of corrugated roofing and be careful to seal the holes with silicone and rubber washers. If the roof angle is significantly different from the preferred inclination (approximately the site’s angle of latitude), make a sub frame to change the array angle. This is also possible if the roof pitch is east/west to point the panels correctly. Be aware that raising the array above the roof surface may subject it to significant wind loads which may damage the array structure or even the roof itself. Pay attention to sealing if penetrating the roof for the cable entry, or route it off the edge and under the eaves (allow a drip loop). Specialised PV cable is UV resistant but standard PVC insulated electrical cable (and conduit) is not. Polyethylene water pipe provides reasonable protection. To span overhead, add some fencing wire and attach at each end to take the strain.
Pole mounting This is fairly simple for a single module, but becomes more structurally challenging for an array of more than 2 modules. It needs more materials than a roof mounting, typically a pole and framework, and concrete for the foundation. Steel water pipe makes a good pole, 2” for a single module, 3” or 4” for larger arrays. Some welding will probably be needed to mount the array framework to the top. The pole base should have some side struts to embed in concrete and provide a secure anchor. Use armoured cable or polypipe to protect buried power lines (at least 300mm deep, preferably 600mm or more). Poles give more flexibility in siting to avoid shade, and can provide more predictability if turning up to install at a site not previously inspected. They can be a good option for structures with soft roofs – thatched huts, tents, etc. They can also be more secure against theft if reasonably high and mounted with tamper-proof fixings. On the down side, they generally cost more and take longer to set up due the foundations.
Ground mounting Costs and installation time are usually higher than for roof mounting. Concrete foundations and metal framework are required (wood is too vulnerable to rot, termites, fire and theft). An alternative approach mounts the modules on purpose-made plastic buckets that are filled with ballast and placed on flat ground. Fencing is advisable to protect the array from stock and people, but should not shade the modules during peak sun hours (9am – 3pm). Ground mounted arrays are common for water pumping systems with large arrays and no buildings. They can also be more suitable for areas with extreme wind conditions. They are easy to clean, but are more vulnerable to shading from both fixed and growing (shrubs, trees, grass) obstacles.
Theft protection Solar panels are a valuable commodity, and in many parts of the world PV theft is a common problem. No system can be made completely theftproof, but some strategies to reduce the risk include: Engagement: if local people feel a sense of involvement, ownership or benefit from the system, they will be more likely to keep an eye on it. Exclusion: site the array away from casual access, in a compound or behind a fence, high on a pole or roof. Secure: use tamperproof fixings that require specific tools; construct frames so that fixings cannot be accessed with adjustable spanners or hacksaws. Supervison: locate the system where there are always people around.
Summary points Locate array near the load but out of shade. Ensure it is pointing the right way and can be accessed for cleaning. It should be structurally secure to resist high wind and casual theft.