Currency, in its many forms, had been part of human civilisation for thousands of years.
A natural evolution of trade from the arduous process of bartering, where trader needed to find an equivalent exchange of goods to complete a transaction.
The avent of metal coins, which had a known and reliable value, allowed trade to flourish.
It facilitied the diversification of jobs and even intvented a new one, the banker.
As trade flourished, and coins were mined the wealthy needed a secure location to keep their money safe.
A SYSTEM BASED ON TRUST
Banks became common place, where vasts amount of amount was stored, and the responsibility of keeping track of it fell into the hands of these bankers. The banker could facilitate trades between people, without the physical coins, from which the currency gained its value, ever needing to be seen. This evolved to the world we now know where electronic and paper money is commonplace, a system based on trust. Trust that this money has intrinsic value, trust that become tarnished by global financial crisis where banker created excessive amount of another new form of currency debt, a currency based on assumed future earnings.
RISE OF SATOSHI
When this system fell part with the crash in property prices, many ran back to the tried and true method of using gold as a safe and reliable currency, resulting in the prices peaking at the height of the crash,
but one clever started to wonder is there better way. That person was allegedly"Satoshi Nakamato" the mysterious founder of the first crytocurrency, BITCOIN.
Satoshi released this statement conveying his motivation for creating bitcoin:
The root problen with conventional currency is all the trust that's required to make it work. The central bank must be trusted not o debase the currency, but the history of fiat currencies is full of breaches of that trust. Bank must be trusted to hold our money and transfer it electronically, but they lend it out in waves of credit bubbles with barely a fraction in reserve. We have to trust them with our privacy, trust them not to let identity thieves drain our accounts. Their massive overhead cost make micropayments impossible>
Crytocurrencies were invented primarily to eliminate the banking system middle men from this trust system. To ensure yge security of people's hard earned money and allow for cheap accessible transactions.
A beautiful idea, that in practice has its flaws.
WHAT IS THE BLOCKCHAIN
In order to eeliminate these middle men, we find to a way of creating a trust system between individual using the currency. A way of ensuring that someone cannot simply write a transaction crediting their account with Bitcoin, without permission.
This were the Blockchain comes in. In its purest form, Bitcoin is a currency that uses a system of complicated keycodes to verify transaction between individuals. Batches of transaction are filed into somthing called a block every 10 minutes.
For Bitcoin the max amount of transaction per block is around 2,400, so Bitcoin has a max transaction speed of 4 Bitcoin per second. Each of these blocks needs to be verified in order to verify the transaction history and crytocurrencies do this in a pretty suprising way. By guesswork.
MEET SHA-256
Bitcoin uses a system called a crytographic hash function, in this case SHA-256, to verify each block and it works like this.
SHA-256 simply outputs a string of 256 bits, that's a 256 long string of 1s and 0s, for a given input. The output seems random, but it's not. SHA-256 will always give the same output for a given input, But, as far as we know, it's impossible to take the output and figure out what the input was.It's a one way street that means, in order to generate a specific desired output, the only way of doing it is by trial and error. Guessing inputs and checking the output and to do this quickly requires a significant amount of computational power.Essentially employing millions of little monkeys in your computers to type number until one manages to get it correctly.
So how does this apply to Blockchain verification of Bitcoin?
For a block to be added to the chain it needs to be signed with a SHA-256 input that will result in a predetermined string of zeros at the start. The number of zeros needed is determined by how much computational power is trying to verify the Blockchain. The more zeros needed, the more computational power is needed.
We want the blockchain to be verified every 10 minutes, so in order to maintain that verification time, the number of zeros needed keeps rising as more computational power id dedicated to the network.
It is through this huge dedication of computation power that ensures security of the Blockchain. It would be unfeasible for an individual to rewrite the Blockchain with false information, as they would first need to dedicate enough power to sign previously written blocks, and manage to keep up witb the blocks currently being written by the rest of the network. A Herculean task.
NEGATIVE SIDE EFFECTS
This Blockchain technology is fascinating and has huge amount of potential outside of just cryptocurrencies, but it has negative side effect, which have been exacerbated by the hijacking of the bitcoin hype train. The Blockchain is verified by miners, these are people who guess the inputs for SHA-256 to generate the desired output, and they are rewarded for doing so with some bitcoin.
To increase your chances of being the first person to correctly guess an appropriate input, you need to maximise your computational power, basically employ more monkeys, but obviously this has a breakeven point.
Computational requirs expensive equipment, and equipment prices have only risen as the demand for Blockchain verification tech increase, and it also requires electricity.
There are giant mining farms in the wastelands of Iceland that use the cheap geothermal energy and the abundance of cool air to minimise their electricity costs for this very reason. But the prices of Bitcoin has inflated so dramatically that it's still profitable to dedicate a huge amount of computation power to mine and it has risen to damaging levels.
One Chinese mining facility was reportedly spending 80,000 dollars a month on electricity, but was turning over 1.5 million per month. With returns like that, it would make prefect sense to expand AMD increase your electricity damands, and this is exactly what wee seeing.
Digiconomist have constructed a Bitcoin energy consumption index which has estimated that the network of computers that verify Bitcoin transactions draw 3.4 Gigawatts(GW) that 3.4GW adds to 30.1 terrawatt hours(TWh) of energy per year. 
This means that VISA has an energy need equal to that of around 17,000 U.S. households. We also know VISA processed 111.2 billion transaction in 2017. Vastly more than the Bitcoin networks 100 million transaction, at a fraction of the cost.
Comparing to VISA maybe unfair, as they require those pesky middle-men that we are striving to eliminate and move towards decentralisation. You should however consider that the brunt of Bitcoin verification is being preformed by individual like me and you, we simply cannot compete witb our consumer level technology but by a small group of large mining facilities. Centralized facilities wherw the power is in the hand of a few.
SO IS THERE ANY FOR BITCOIN?
We need to consider tgat the system is not in equilibrium. The Bitcoin network is limited to a production of 21 million coins. Overtime there will be fewer Bitcoin left to mine, a feature of original plan by Bitcoin's mysterious founder Nakamoto. Once the majority of Bitcoin have been mined the block reward will become an insignificant percentage of miners overall earnings. Instead, miners will get their reward from small transaction fees, which are already part of network. So in equilibrium, the energy demanded by the network will only be driven by these small transaction fees, which is currently significantly below coin-value . Making it even less profitable to flood the market with computational power. This where come to our next point in why Bitcoin isn't working?
This vast majority of transaction occuring for Bitcoin are not to buy goods and services, as currency was invented for, but are simply trade of currency.Swapping traditional currency for a share of TGE Bitcoin on the market. This may sound familiar to you, because it's exactly what gold is used for. This is not what Bitcoin was created for. Bitcoin was created so that you would never have to use dollars again. That you would trade your dollars for Bitcoins, and never trade back. But Bitcoin is too volatile for anyone to use it like that.
Bitcoin has become incredible volatile investment vehicle, witb thousands of people attempting to trade on speculation and achieve this dream of becoming Bitcoin millionaire. But in order for a currency to work in society it needs stability, people needs to trust that the money they own today will be worth same amount tomorrow, this is why the dollar is so successful. Several countries around the world use the dollar because their own currency has become so volatile no one can put their trust in it. How can we expect Bitcoin to become a useful currency when it's value jumps so dramatically between dates due to irrational speculation, and market manipulation.
For now Bitcoin is largely a useless environment damaging investment vehicle, which was not the vision Satoshi had for it. Even if the Bitcoin prices manage to stabilize as equilibrium is reached, it's limited transaction time of 4 transaction per second will likely prevent it from ever becoming a useful currency, and will likely become a wealth storage method similar to gold. Blockchain technology and cryptocurrency are a fantastic idea in theory. Just as paper money and debt are, but they're flawed for the same reason PEOPLE.
If there are ways of exploiting a system for financial gain, people will find it. The only way stopping that is setup systems to prevent exploiting, and this is what other currencies like Ethereum are trying to do, while bluiding upon the foundation Bitcoin laid, but if history has proven anything, if there are way of exploiting a system for financial gain people will find it.