Across most of the world, the majority of domestic houses and hotels ect have been sold a pressurised hot water only tank made from many types of steel material.
Being British, the majority of UK homes were installed with copper insulated tanks, open vented with a 40 gall cold water storage tank in the loft area, as a guaranteed cold and hot water supply.
Being open vented and not as today being unvented and made from stainless steel, is 100% okay if the hot water tank is heated by a gas/oil boiler or an electric heated, which is set to switch off when the temperature set on the tank thermostat has reached its set point.
What the solar collector industry has done, is to adapt its collector to a standard hot water tank, with a solar coil added to the gas boiler coil as the easy way to sell a solar water heating system, but all we have done is created more problems, as the only way you can shut off the sun over a 12 hr sunny day is through the controller. What happens then is the closed pressure loop of the solar part stagnates.
On researching all of this and not being from the plumbing and heating industry, I started to develop in 1990 a open vented copper tank, but with a pipe in pipe coil in the tank, which provided hot water at mains pressure, while the solar part was vented as was the tank.
Twenty five years on, our range of Multi fuel thermal stores have been copied by many of the major UK tank manufactures, while our commercial tanks have been developed as flat pack, from Sth Africa, USA and this year from China.
My Uk home uses flat plate heat exchangers common across the world used in gas/oil boilers and for comercial use.
Unike the global solar water heating industry including China, every time a solar system is exported and installed with a pressure closed loop solar tank, servicing and call outs to the owners house will continue for ever and a day.
Very few solar collector/system manufactures understand how a thermal store works, which is why sales in SWH continue to fall, while solar PV and PVT is expanding as can now heat your hot water tank through the electric heater.
But none of these systems can provide central heating support as my house has been doing for the past 12 years. PV sales cannot heat millions of galls of water for comercial use, like hotels, resturants and the food industry.
To change this industry strangle hold, their needs to be existing companies willing to invest in marketing this proven concept