Combined solar + heat pump water heater
 

A while ago I was given an office acclimatisation combined cooling/heating unit. It was interesting as it had heating via a water circuit and cooling with a refrigeration circuit, the heat being supplied or removed in the water as necessary.

The unit had a tube-in-tube heat exchanger that I thought might come in useful.

Having installed a solar hot water system with a single panel, 160L tank etc. which provides hot water from roughly the end of February until the end of October, I decided to add a heat pump to create a hybrid system and therefore benefit from the best of both and (hopefully) have the most efficient hot water system possible. The heat exchanger, being flat and having the two required circuits was perfect.

A schematic of the system is shown in the photos below. The 3-way valve controls the circulation of the mixed antifreeze/water through either the solar panel or the heat pump heat exchanger.

In the photos you can see the solar panel, the non-pressurised fill valve and auto purge valve for any air in the system. When the system is filled (unpressurised) with antifreeze the valve is closed and the system is then pressurised via another valve connected to the pressurised cold water.

In the background you can see the heat pump (it's not the Panasonic one in the foreground).

I haven't any photos of the tube-in-tube heat exchanger without insulation so I've drawn on the relevant flows in blue for water and yellow for refrigerant. In the same photo you can see the 3-way valve.

In true eco tradition the solar panel was a ruptured unit recovered from a customer (cost 0 but it did cost me 150 euros for a new laminated glass panel), the heat pump (air conditioner) was recovered from another job where it was replaced with a larger unit (it was old but functional, again cost 0), the expansion vessel was given to me, the circulating pump was recovered from a scrapped boiler, the tube-in-tube heat exchanger was recovered from the office unit (cost 0), the rest I had to buy although the 160L tank was a necessary replacement as my old tank had failed.

I'm very curious about a similar solar heat system: the vacuum tube systems are supposed to be very efficient (up to ~80% maybe?) and if the storage tank was warm enough, it could be used as a hot water heating system for a house. Or, there could be a heat pump to boost the temperature, and use the tank as heat storage for overnight and cloudy days. Domestic hot water could be either directly off the solar heat tubes or also use the heat pump.

Has anybody run the numbers on this?

There is a German company that sells solar kit that has a dual tank with twin heat exchangers. The idea is the solar panels can heat the water or, when there is no sun it can be heated by a more conventional source (boiler of some sort).

The tank is split into two, a smaller tank at the top (around 150-200 litres if I remember) that is enamel coated and used for domestic hot water, and a larger tank underneath (800 litres or so) that is heated by the two heat exchangers and is used for heating the house.

DHW and heating water obviously need to be separated from each other.

My system is a hybrid as it uses the same heat exchanger coil in the tank, the heat source being determined by the 3-way valve.

A project I have pending is a hybrid heating system using solar panels, heat pump and 2 hot water storage tanks. I'm just finalising the design.

Right, that sounds very close to what I am hoping to find. I would just add a large enough tank to hold enough heat to 'bridge' the longest dark period and/or the capacity of the vacuum tubes to gather heat.

Combine it with a few solar PV panels, and you can have a carbon-free heating system.

You guys in Europe have a lot better stuff than we can get here. The market is bigger and we have archaic regulations on tank and approvals. We would never get a split tank approved here for a reasonable price.

That said, there are lots of dual coil tanks available here but only up to 500L, Viessmann, Bradford-White, Stiebel-Eltron, etc.

Randon and I have made our own special purpose tanks but we both have full shops.