4 Selection of energy sources for objects

Use and practice are, of course, of first importance for the functional and ergonomic definition of the object, but it corresponds, as previously seen, to energy needs. To make the choice of one or more possible sources, the essential criteria are instant electric power, the energy is then a combination of the average values of power and autonomies for user profiles. So, we will deal with both aspects.

4.1 Micropower objects: nW to ¼ W

These communicating objects do not give present problems, as, for example, the RF ID labels, self powered by EM field of RF ID readers. In the field of nano-technologies (eg medicine, ADN processor), MIT (Massachusetts Institute of Technology) has studied low speed processors that consume less than 1 ¼ W, which offers several possibilities of powering solar cells adapted to artificial light.

nW or ¼ W: Full autonomy is easy to obtain with an integrated source (battery cell, solar cell , inductive interface, etc).

4.2 Low power: ¼ W or mW

Electricity consumption is generally some mW to some tenths of mW, but this excludes functions like a fast backlighted colour screen (ex: calculator, wireless mouse, sensors, radio). In this case, primary batteries suit. For some mW, it is possible to reduce the consumption of the battery using a solar cell, but the object must be close to a window. The artificial light does not suit this level of power. The battery might be replaced by a small capacitor or ultra -capacitor of several farad.

¼ W to mW: One can target the full autonomy of several years with primary batteries with low self- discharge or a rechargeable battery or capacitor (ultra-capacitors) associated with photovolta c cells which avoids replacement of battery.

4.3 Average consumption: mW or W

This range of power corresponds to objects equipped with a low speed processor (< 30 MHz), a short distance radio link, a LCD screen, that often offer autonomy of some days or even weeks. These frequently used objects (PDA, DECT, remote commands) are generally equipped with rechargeable batteries (lower running cost than primary cells), with higher possible capacity in a given space not to recharge too often.

The necessity of some functions like synchronisation has the huge advantage that we naturally dock the object for communication and recharge. The recharge is rather slow (some hours) but the object usually remains for a long time on the dock.

As these functions tends to be performed with a wireless link, the necessity to dock the object might decrease, which is a problem for the recharge. Fortunately, one of the good reasons to dock anyway is perhaps to put it away (like with a DECT).

Good autonomy is possible with primary batteries but solely for intermittent use objects. Accumulators are of much lower cost but need to be recharged. To dock some objects (PDA, DECT) to recharge or to put away is a transparent operation that eases recharge.

4.4 Highpower: Ws to tens of W

This concerns equipment equipped with powerful processors, large backlighted screen, motors, lasers (for ex. audio readers, motorised webcam, robots).

Consumption is very variable due to considerable power changes. The energy management must be treated with great care because in active mode the autonomy is very limited. Rechargeable batteries associated with a fast charger are a solution.

The compatibility of size between primary cells and rechargeable batteries is sometime possible as well as the association between a rechargeable battery and other power sources (solar panel, fuel cells, manual dynamo, low voltage plug etc).

An autonomy of some hours provides the choice of rechargeable cells and a simplified charge (dock, fast charge on mains, low voltage local network).