Please let me try to elaborate:
I'm not really looking for a cheap payment service in itself, otherwise Paypal etc would be the easiest as you stated.
A project example that I could imagine would be to implement energy efficient technologies in 100,000 buildings in Africa.
Each building that is interested has to sign a smart contract on a blockchain and gets the technology automatically delivered.
As part of this contract, a micro-payment is automatically conducted to a specified address and if the payment is not received in time, the new technology stops working until it receives the correct payment amount again. So it's not only about the payment but includes several automated steps which reduces the resources spend significantly. The automated signature and implementation of the contract is important. And the link of payments and technology functionality is also important as an enforcement mechanism.
Is this something that could be done and is realistic on a blockchain? I understand that this is what you mean with "a self sustaining system"?
Yes, this is very possible with a "blockchain" with smart contracting. But, it is also possible
without a "blockchain". So the question is really about using an altcoin that already exists
and be subject to that, or create your own "ledger with contracting abilities" that your
business or the UN will maintain, in all aspects, in their own servers.
The reason why I originally answered your question the way I did was that as a
business, you would need to decide if you are willing to outsource all the backbone aspects
of your energy efficient business to an altcoin blockchain (Like Ethereum) that would
maintain and provide a majority of what you are looking for. The issue isn't so much about
whether a "blockchain" can provide what you are looking for, it is whether you wish to rely
only upon it and whether you really need what it was originally designed to provide.
Also, there are many complex question and scenarios that need to be answered to determine
what type of "blockchain like" system you would need. For example, will you be able to make
edits to the data in your "blockchain"? Will you need a master key backdoor to the "blockchain"
so you can turn on electricity for some customers, like hospitals, even though they did not make
their payments on time? Will you require these customer buy your own token, a currently
known and widely used token (bitcoin or altcoin), or use a regulated payment processor? And
so on and so forth. There are many questions that need to be determined first to prevent
issues later.
You need to keep in mind that if you use an altcoin blockchain, you will and your customers
will be subject to any issues/bugs/failures/rollbacks/attacks that could occur with that
blockchain and its token. In this sense, you have outsourced your security/finances/etc to an
outside unregulated entity, but any liabilities your business may incur will be attributed to you
(unless you than individually sue the developers of that altcoin for some failure, which in some
specific circumstances, is essentially impossible or too complex to bother with). Point is, there
are many things the "blockchain" provides for "near free" (not really near free, but for most
laypeople, it seems to be) but it comes at costs that are not fully apparent.
The cost saving or cheaper aspects of using a "blockchain" is an illusion. In fact, it is very
expensive. What most do not understand is that the costs of those "cheap transactions"
are actually distributed both throughout the node network and applied directly into the
token itself (besides production costs). The secret was, as the token increases in value,
it is actually a representation of the expenses and burden on the system, being applied
directly into the token itself. So, when people want to use a "blockchain" since it is
"cheaper", in reality they need all parts of Satoshi's system to actually be sustainable
and secure (miners, blockchain, and token). As the expense of the "blockchain"
increases over time, so does the token's growth in "value/expense". So, in theory,
"blockchains" are only cheaper as long as the customer base increases beyond the token's
accumulated expense. (As a side, IMO if you can mitigate or slow those expenses, you
will not need unlimited customers into the future (ponzi) and cause premature system
failure. The system should always self balance and not fall into ponzi status as long as
divisibility of the token is infinite.)
When I said a "self sustaining system" I am actually referring to something much more
complex. The "blockchain" is only a small part of the whole system that Satoshi created.
For a simplistic breakdown, the "Miners" build the "blockchain", the "blockchain" issues
and records the "tokens", the "tokens" are traded for "value" either P2P or on exchanges,
the "value" incentivizes the "Miners", and than it repeats over again. This whole system
is a self sustaining system that reinforces security and other interesting things. The
"blockchain" is only the part that facilitates this payment system in a decentralized and
individually provable way. In theory, if you changed out the "blockchain" part with a
single or few servers, it would still work, but would be centralized and subject to
governmental regulations.
So, is it possible to use a "blockchain" to facilitate your example, the simple answer is yes.
The important question is whether you wish to outsource and be dependent upon an altcoin
or build your own system, which will when completed, be no different than what people
used before the "blockchain" was invented.
edit: typos and repositioned paragraphs