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Stand Alone Solar System Design – Off Grid Solar

Date: May 3, 2018

How to increase efficiencies of a Stand Alone Power System (Off-Grid solar), aspects on how to improve performance and ways to reduce the costs of Stand Alone Solar Power Systems.

The selection of appliances;

Obviously, a premises which has more efficient appliances such as energy efficient refrigeration, LED and/or compact fluorescent lights, LCD televisions, energy star rated dishwashers and other appliances will not require an Off Grid solar system equal in capacity than one which relies on less efficient appliances.

Appliances which are efficient will typically require an Off Grid solar system of less capacity including the solar array, battery bank, solar charger and inverter than one without efficient appliances. One of the major aspects of designing an Off Grid solar system is that it actually may be more cost effective to replace an inefficient refrigerator or air conditioner than installing a larger system.

The Battery Bank;

The article regarding Sealed deep-cycle batteries versus open-vented deep-cycle batteries discusses advantages and disadvantages of both. The cost per annum is relatively comparable, both are reliable, both offer a good life-cycle in terms of their cost, but the decision when all this is weighed up is what the user can ultimately afford.

Furthermore, a battery bank will increase in the amount of ‘cycles’ if they are maintained in a stable climate, that is they absorb charge (recharge) more efficiently at temperatures around 25´C. A battery bank located indoors, in a garage or shed will provide a stable environment as opposed to located adjacent a house or outdoors etc.

Also an increase or decrease in temperature beyond 25´C of the battery bank will also effect the ‘State of Charge’, that is the capacity of the battery will decrease in both of these scenarios. Therefore, where a battery bank temperature remains constant it will increase the life expectancy of the battery bank in an Off Grid solar power system.

The Solar Array;

While monocrystalline and polycrystalline solar panels have both advantages and disadvantages. As a general rule of thumb, mono crystalline panels do have slightly higher efficiencies than polycrystalline. The most pertinent point in relation to maximising the efficiency of a solar array is the positioning. A solar array will perform at its maximum efficiency when facing True North, other than a solar array on a tracking device which allows the array to follow the azimuth of the sun.

As a general rule of thumb, the solar array should also be tilted to same angle as the latitude angle i.e a solar array at a latitude angle of 30´ should be tilted at 30´. It may also be maximised more by tilting further towards the winter angle, however this is a design decision which largely depends on the users demand.

Furthermore, to gain the maximum performance from any solar array or Off Grid solar power system for that matter, it should be positioned where there is no effect of shading at anytime of the day, if possible. They should be fixed approximately 100mm above the roof on a flushed mounted type system to reduce the effects of temperature.

Distances between solar array, controls and battery bank;

Typically, long distances between a solar array and charge controllers, between battery banks and inverters requires larger cable sizes. The expense of larger cabling between any of sets of components can add a significant cost to an Off Grid solar power system. Furthermore, longer distances also require additional labour expenses.

There is some helpful information in the pages regarding Maximum Power Point tracking versus Switched Regulation and also in AC Coupling versus DC Coupling which may assist in which technologies are suited depending on the application. However, distances between components should be ideally kept to a minimum in order to reduce voltage drop across distances and to avoid additional costs of larger gauged wires and labour expenses.

The inverter;

Off Grid solar Inverters are both designed to suit the maximum demand and the average demand of the user. Inverters typically have a surge rating, that is they are able to provide a short peak of electricity to assist electrical demands from appliances that require a surge i.e. induction motors typically found in refrigerators, washing machines, pumps, power tools etc.

Off Grid solar Inverters typically have an average maximum efficiency which is usually below their maximum continuous rating. So although an inverter should ideally be selected to suit both the maximum and average demand, it should also be selected to suit the maximum efficiency of the demand, where possible.

An inverters average efficiency is found somewhere between 85-90% of its continuous rating, therefore an inverter should ideally be selected slightly higher than the users continuous demand.

The Solar Charger (Maximum Power Point Tracker/Switched Regulator);

MPPTs do convert more usable electricity from the solar array into battery recharging and similarly to electrical loads and since that the additional cost may be less than the cost of an additional solar panel, it may be worthwhile in purchasing a MPPT over a switched solar regulator.

Many manufacturers’ claim MPPT’s gain up to 30% more power than from conventional switched solar regulators (PWM). However due to temperature derating of solar panels typically on a summers day, the losses can be higher than a 15%. Again, a MPPT can accept a higher voltage input, so the conversion to the battery voltage is more favourable than a switched regulator and  ultimately depends on the type of application.

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