How to prevent overheating?

Welcome to the forum.
I know nothing about solar water heating but it seems a very simple remedy would be to cover part of the panel. If it works you could experiment with more or less coverage to suit your needs.

1.) Get a larger water tank, or,

2.) Get a second (larger) water tank and plumb it between the existing water tank and the collector(s). Make that the solar loop and feed the existing 30 gal. tank from the other tank, or,

3.) Reorientate the collector(s) so that the tilt is somewhere between 45 and 60 degrees to the horizontal. That'll decrease the intercepted irradiance during the summer and increase it during the winter. Result: reduced summer max. water temp., increased winter max. water temp.

4.) Cheapest, and easiest but probably the most visually offensive and involves a bit of work:
Go to big box hardware and get the necessary number of corrugated fiberglass roofing panels. Mine are gray and probably the mostly visually unobtrusive, but the color won't affect the performance curtailment.
Cut them to the size of the collector(s) and the strap them to the collector(s) with (black) bungee cords in a way that makes for easy removal/reinstall of the corrugated panel(s) 2X/year.
Store the fiberglass panels in a shady spot during the winter.
The fiberglass panels last about 5 years, the bungee cords about 2.

I too have a direct system.
This is what I do from about early March until maybe early November or so.
The method is fit for purpose and gets me a solar thermal water heating fraction of ~ > 0.95 or so without summer overheating, but my solar thermal water system is relatively accessible.

Best option would be an electronic temperature sensor board and an electric water valve.

and
Best option would be an electronic temperature sensor board and an electric water valve.

Thanks for the suggestions.
Panel is on inaccessible roof. House is in hurricane zone. Electronics are not good in extremely rural and salt-air zones.

Are you talking about a solenoid valve and a high limit shutoff ?

If so, most differential controllers have high limit switches that turn pumps off at maybe 160 F.
Sounds like maybe the OP's isn't set right, has a failed sensor or two or the whole controller is toast.
My controller has a high limit control that's adjustable between 120 and 200 F.
I've also got a solenoid valve that's closed if the pump is not activated but since check valves don't work in any real sense for more than a short time, its primary function is to act in place of a check valve to prevent thermosiphoning.

So, do you have a differential controller ? If not, is this a thermosiphon (gravity) type heater ?

is this a DIY system ?

I have a small PV panel next to the solar thermal. It runs a low voltage pump (El Sid) that circulates the water through the solar thermal panel when the sun is up, and doesn't when the sun is down - clever. No this is a professional installation. No controller. Not thermosiphon - see PV/pump description above.

At this time, since covering part of the collector system seems to be out, my guess is your best bet would be to either get a holding tank and so increase the system volume or, skip the holding tank and decrease the collector area if you have more than one collector.

But if you get a holding tank, you'll need to estimate the system output so you can size the tank. Too big and you'll wind up with tepid water (which may be OK if it's only to preheat water and then boost it the rest of the way with electricity or some fossil fuel fired device).
Or, too small a tank and you may not eliminate the whole problem.

For as (in)efficient as this system probably is, you may be $$ ahead not to mention grief with a PV system that meets some of your household electrical needs and also heats your potable water using a conventional electricity fired tank type water heater.

Only one collector.

Thanks for the PV suggestion, which does make sense, except PV connection to the grid is prohibited where I am. So it would have to be entirely stand-alone and probably not worth the capital and labor expenditure.

The "second holding tank" option sounds promising, another 30 gallon one, and I wonder if it is possible to plumb it so it's active (in-line) in the summer and inactive (drained, bypassed) in the winter?

You're welcome. Where do you live that prohibits grid tie PV ?

Most any plumbing scheme is possible and usually not a problem with a bit of thought by experienced pros. Just use licensed professionals and check with the AHJ for potential unknown and quirky problems (such as cleaning seasonally unused tanks).
Having done it for going on 17 years at current residence, if any way possible, I'd still consider covering a portion of the collector surface. Cheap, easy and flexible. Just sayin'.

BTW, just curious, your screen name says you're in Boston, but the system's in FL ?

Yes, snowbird - Boston summer, South in the winter. Quest que c'est AHJ? The rural "Global South" is not as technologically advanced as the rest of the world and monopoly electric companies in small communities can do what they want. Like prohibiting grid tie PV. Which might actually make business sense if some idiot does a DIY grid tie that f*&#s up the grid...

I really think a low-temp (150F) T&P is the best solution but I can't find one. Best I have found is a pressure-washer overtemp device but it has a plastic barb fitting on the outlet that I am very reluctant to incorporate in my home. Any thoughts on customizing the opening temp on a T&P. The other option I would like to use is an automotive thermostat which has the correct opening temp but I can't figure out how to package it for a residential water system. They have rubber hose and hose clamp packaging.

AHJ == Authority Having Jurisdiction.

Usually "the building inspector" or the county or city building inspection dept. - the folks you go to when you need a building permit and also the ones who tell you that you can't do something stupid but can't always explain why it would be dangerous to do so.

As for pressure and temp. relief valves with a lower temperature rating, I've never seen or used or specified such devices but I've been told they exist, mostly for plumbing systems with plastic components. I believe relief valves with pressure and temp. ratings of 100 PSI(G) and 180 F are about as low as you'll find.

Look Guido, I'm not trying to be more Catholic than the Pope here but having been around lots of industrial and commercial energy systems and designed my share of them and seen what can happen when people do dumb and sometimes dangerous chit out of ignorance, laziness or hubris, I've found it safest to not get too fancy or reinvent the wheel, especially when safety is involved. and if you don't think safety is involved in relief valve matters, then you're out on the ignorance limb with a saw.

What you have is a system that has either too small a tank for the collector surface or too large a collector surface for the tank or a collector array that's in need of a higher tilt to avoid summer overheating or a lot of other things. Without seeing it, it sounds like the system design was not well thought out or somehow changed the design after installation.

In any case the last thing I'd suggest is messing around with the relieving devices on the system beyond ensuring they are working as intended and are fit for the intended service.

IMO and meant respectfully, but as a retired P.E. who designed a lot of refinery and power plant equipment, I'd humbly suggest you get some advice from someone who's competent and experienced in solar thermal design and pressure vessel technology to help you sort out what you have and what you want to do.
Without such advice, I see the probability of something going wrong as being greater than I'd be willing to accept.

Take what you want of the above. Scrap the rest.

Thanks for the response JPM, well thought out and communicated. AHJ is a joke in this locale.

I was not intending to remove the existing T&P if that's what you thought. Retired P.E. here as well, small world (medical device/product design, not plumbing).

Lacking a commercially available low-temp T&P, or an NPT- or sweat-fitting compatible packaging for an automotive thermostat, I probably will add another HW tank and hope for the best. Will be a project to figure out how to isolate the second tank when it's not needed in the winter with the fewest number of ball valves.

You're most welcome.

I'd expect 3 valves plus a couple of drain cocks ought to do it but I'd still respectfully suggest getting a licensed plumber to do the work and also run it by the AHJ before you start.
Just don't skimp on valve quality - that's little more than foot shooting false economy.
While you're at it, add a check valve that works or at least change out the one you may have. If it's more than a couple of years old, you'll most likely find that it's failed partially open from fouling due to mineral deposits and become useless.

Yea, a lot of folks are not favorably inclined toward the AHJ's and often look n them as an impediment to progress. Unfortunately or not, if/when a disaster takes a property or any part of it, insurance carriers usually look for unpermitted improvements as a possible reason to deny a claim.
More just sayin' stuff.
Besides, a lot of times AHJ's do actually know some stuff, like most jurisdictions not liking non-code or otherwise inappropriate stuff from the automotive world for residential service that probably isn't ASME rated and probably also not suitable for potable water service.
A lot of times they can be a source of probably correct, pertinent and practical information.
After dealing with all kinds of code inspectors, including my own QC people, I found an inspector is like a Marine in at least one respect: the inspector can be your best friend or your worst enemy and you get to choose which.

Off topic, my wife's a retired Nurse-Practitioner who specialized in heart transplant and early lvad implementation and applications. She got to work with a lot of manufacturers and designers of medical equipment back in the early days of Lvad's.
We've had some interesting discussions over the years about the similarities between fluid mechanical systems and the human cardio-vascular system. We've both learned a lot from the discussions.
WAY back in the day (as an advisor-assigned side project during grad school days) I got to contribute to the design of one of the first blood heat exchangers used in open heart surgery applications. My part was as one member of the team that did some fairly extensive measurement work on the transport properties of human blood which, curiously enough, had not been extensively investigated up to that time, but essential to heat exchanger design. Most of it was pretty boring but one interesting thing we found and quantified - that was pretty much already known but had not yet been well quantified for heat exchanger design - was that human blood is thixotropic. Fun stuff that with 5 bucks will now buy me a cheap Starbucks.
FWIW, I believe the result of all our work might still be on display in the SUNYAB engineering library.