So I have (or will):

Super insulated house
Passive solar design
6-8m2 of Evac tubes
Woodburner with backboiler 70/30 boiler/room
No gas boiler

Clearly the main goal is DHW, ETs will take care of Sunny days and woodburner on cloudy days.

Question is do I incorporate it all with:

A) DPS or similar 300-400 thermal store/heat bank, w/ flat plate heat exchanger and immersion backup

B) 300l Direct 'traditional' cylinder but with solar and WBS coils, immersion backup, WBS thermo-syphoning, thermostatic mixer on hot water outlet.

Cash is tight but simple reliable robustness is also desired.

Pros and cons appreciated, which is cheaper, which is more reliable, which is more efficient in summer, which is better in winter (the latter naturally more important).

J

No CH required, but incidently what's Phex?

J

Ahh...

DPS do two distinct types of store: Pandora which are coil heated and GX which are directly heated. Latter is the only way that you can use solar from your panels for CH but looking at your spec that's not relevant. I think it depends where you can put your cylinder and whether that gives enough hydraulic head to give acceptable DHW flow rates. Not sure what you meant by B) direct - with WBS and solar coils it will be indirect or did I miss the point. Assuming you can fit your tank in your roofspace, which is desirable with roof mounted panels anyway to minimise pipe runs then it could be that a basic tank with 2 coils is the way to go, certainly the simplest and cheapest.
Mike

This is a tricky question, which I have considered myself at length and not yet decided. Just to complicate matters there are variations on your options which would improve efficiency, such as stratifier solar input (rather than via fixed-height coil).

Some pros/cons:
Thermal store needs to be hotter for given output temp at shower/taps, due to drop across heat exchanger. Allow 5C, but it could be a bit more. So that means in say october a conventional vented tank will still get hot enough for a shower by solar alone. In a heat bank it would be too cool.

Heat/bank T-store provides mains hot water pressure, as does unvented cyclinder. open-vented cylinder does not.

Conventional tank is cheaper (no thermal store coil or heat-bank PHE/pump/switch).

Thermal store has no corrosion issues due to all contained water being treated. This allows them to be built of cheaper materials (Steel).

Hard water scale issues are different: on heatbank/store it can happen in PHE/coil, on conventional it collects in bottom of tank or maybe on outside of coils or in PHE of solar input.

Heat-bank/Store is more legionella-proof, as sitting water is treated.

Conventional store more reliable (no flowswitch/pump to go wrong). Coil-based thermal stores avoid this problem but scaling can be an issue as can overpressure if your design has a copper DHW coil (remember to include small EV and pressure-reduction valve in this case).

Posh thermal stores are generally much better designed and thus more efficient - stratifiers on solar input and CH return, diffusers on inputs, thicker insulation etc. But all of this tends to make them more expensive too.

Heat bank/T-sore alows direct boiler/wood-burner connections, which is more efficienct than indirect via coil.

Direct heat-inputs allow 'smart tank' where hot water fills from top down, rather than coil-up.
Heat-bank/T-store is easier to load in a stratified fashion. However pumped direct connections (such as PHE in heat bank) can easily destroy stratification (if flow-rate too high/diffuser insufficient/no stratifier on warm return).

Overall efficiency winter/summer is impossible to say - its a complex combination of overall system design aspects, inlcuding controls as well as the tank itself.

This paper on the principles of good/bad solar system tanks may help you decide (or may just confuse further): http://www.preheat.org/fileadmin/preheat/documents/intersolar/Simon_Furbo_Heat_storage_for_solar_heating_systems.pdf

All in all it's complicated.

Note that DPS make nice tanks and are extremely versatile, but their insulation spec is poor (40mm). 75mm should be a minimum on a store, 100mm is more sensible. Number and detail design of pipe inputs, especially to upper part of tank, can have a major effect on its heat-loss rate (each is a thermal bridge).

If it has to fit in a UK airing cupboard then that causes design compromises (such as insulation thickness, tank size and weight).

I've probably forgotten some other considerations.

Aha, I was thinking "Direct" in terms of directly heated by heat sources (other than solar), and indirect where all heat sources have a coil in the tank.

I have to say with my DPS store, the temperature drop across the heat exchanger must be quite low, certainly less than 5degrees.

Again, with the DPS store its not just the insulation on the tank, its insulating the plethora of pipes. Especially the heat exchanger. Definitely worth planning for this at the outset. External heat exchangers definitely have some advantages, not least their heat transfer power and service-ability, but it would be nice if the pump could be speed controlled - as discussed in a previous post.

Mike