I agree with both Ugo and Kamal that it will be a very slow process and expensive as well. 

However, for new stand alone products (i.e. emergency lighting) that take advantage of Renewable Sources of energy (Solar, Wind etc) to power and recharge batteries then it makes more sense to use DC power. Elimination of the rectifier mainly lowers the production costs and also increases system efficiency. 

I agree with Ugo (''For example in domestic application it's reasonable to envision a scenario where DC distribution will be implemented concurrently with AC distribution: in this scenario new houses will have some AC sockets (for example for ovens, stoves, etc) and some DC sockets for DC loads.'').

The LVAC system is irreplaceable but implementation of LVDC can be expected in near future.

There are still some technical and financial problems...

If we are talking about co-existence of LVDC and LVAC, I think it is OK. But replacement of LVAC by LVDC?...... I have my own doubts.

No doubt LVDC has some advantages over LVAC in some aspects. But definitely not in all aspects.

What about the robustness of the AC machines?

For example can a DC chopper used for voltage level conversion be as robust as a transformer? 

Can we imagine a domestic fan, for example in  a country like India where it runs continuously for months together without switching off ,be replaced by a DC motor fan? Definitely the maintenance of the DC motor is an affair compared to that of an AC motor.

When LVDC is not a replacement for LVAC, then why go for LVDC distribution ?

Yes. LVDC definitely finds appreciation in locally distributed loads supplied by stand alone sources like Solar. But what percentage of the total global loads are being or will be supplied by stand alone sources?

Yes, I'm with K. Prakash Kumar on this. I assume that LVDC would be used for home and office type appliances. But if there's any more efficiency to be realized, then we would have to agree on one voltage level to fit all applications. We know that can't happen. With AC, local transformers, sometimes very small ones, are used to match the needs of individual devices. Doing this voltage adjustment with DC would most likely incur greater losses than doing so with transformers, and also higher cost.

And then there's the use of the AC frequency to keep time, whether for a clock or for a synchronous motor. With DC, you wouldn't have that useful and cheap bonus feature.

I think perhaps LVDC as a final voltage for use in homes may happen for specific purposes. For example, at work, we have a couple of USB outlets on our desks, alongside the AC outlets. The USB outlets wouldn't be expected to be general purpose, though, right? I wouldn't expect to be able to plug a vacuum cleaner into a USB outlet, anytime soon.

AC domestic fans using BLDC motors are now available in Indian market, a bit costly. I think DC versions of fan using BLDC motors for various voltage supply options are also present. I see a very bright and efficient and sustainable future scenario.

I'm not sure how that BLDC motor scenario plays out, in truth. BLDC motors are actually AC motors, where the DC is converted to some type of alternating current (need not be sinusoid) by an integrated power supply. The advantage is that one can vary the frequency, to achieve efficient synchronous motors, brushless, and vary the speed by varying the frequency. Very good technique.

But I'm not convinced that feeding that internal power converter with some kind of DC, as opposed to AC, is necessarily going to make the process more efficient or cheaper. This is actually the opposite scenario from the one described by Ugo above, in which the appliance uses DC internally. In this case, the appliance ends up having to create AC for itself.

A reconfirmation of "albert" comment regarding Using BLDC.

Mr Kumar's comment was on using AC motor instead of DC Motors. As Albert also specified in his comment the new motors all use some kind of power electronics in their structure (SRM, BLDC, ..) all of these special motors have good performance and will be used more and more in domestic applications and all of them are adapted with DC power.

Dear  Ebrahim,

besides all that was said about efficiency and possibilities of converting power for a dedicated application, one more aspect should be considered when using DC as a general supply.

With AC, there is current zero crossing once every 10ms @ 50Hz, making protection devices small, compact, reliable and cheap.

With DC-Supply >200Vdc, arcing in case of failure becomes a real critical issue and any triggered mechanical device to achieve proper protection levels becomes big and massively expansive. Besides the advantage of gain in efficiency, a conversion will most likely not take place as long as the cost for the DC-system are higher than the ones for an AC-System.

In response to Martin's Comment, I should just add a remark:

I agree with Martin that MCCB in DC circuits are much more expensive than in AC circuit. but recently it is possible to use DC static switches with the possibility to limit the Dc Short Circuit Current. As they are not mechanical, the price is decreasing gradually as in the case of other power electronics applications. So in a near future, such issues are not as today we are  encountering. 

Hi Ebrahim,

valid point :-)

The issue with protection devices based on semiconductor remains a more bureaucratic one.

Several standards demand that protective switches need to provide a safety gap that is characterized in millimeter instead of a blocking voltage capability - returning them to remain mechanical until the standards will change.

But changing international standards is a hazard. Usually a long-term hazard...

A brush less DC motor can be very well used instead of  AC motor...... In my view the investment can also be recovered back(Return over investment) like for example if the complete system is DC and obviously there will be lesser losses in the D-system and hence the distributor can sell power at cheaper price so even then the efficiency of the motor is less the the cost we pay for kwh is less and hence there can be a compromise......