What is a closed currency

Is there already a closed cash flow cycle for the currency BTC?

Closed or open loop is not a black or white definition. For example, even in the United States, you have a significant pace with companies buying parts in other currencies (from other countries), consumers importing overseas-made goods, and investors buying foreign stocks.

So there is no such thing as a fully closed loop. I think you are asking what can be done to keep transactions in BTC either permanently or for longer (before redeeming them in fiat). Unfortunately, it's a slow process. There is no grand over arching plan, rather each individual seller increases the potential for the currency to overturn prior to exchanging.

I see three main factors that determine how likely it is that buyers of BTC will keep the BTC above selling for fiat currencies: 1) Availability of goods. The more goods and services that are available in BTC, the more valuable the currency is relative to fiat currencies and the more likely someone who purchases BTC will keep it.

2) Value of goods. When goods in BTC are sold at a discount, the value of BTC increases. When goods are sold in BTC at a premium that reduces the value of BTC.

3) price stability. The more volatile the price, the greater the risk of losing purchasing power by holding BTC. This reduces the likelihood that an acquirer of BTC will hold that BTC over more stable currencies.

There are no easy answers to the problem of extending the BTC loop because BTC is not a planned economy. As an individual, your best bet is to sell goods or services in BTC and look for vendors who do the same.