The pros, cons and process of installing solar thermal panels 

We shine a light on the pros, cons and process of installing solar thermal panels as an alternative to trending solar PV panels
 The pros, cons and process of installing solar thermal panels 

Your thermostat/controller will be detailed to use the solar thermal water heating to augment your hot water heating needs throughout the day. By switching some hot-feed appliance use and showering to the late afternoon and early evening, you’ll make best use of your solar thermal’s contribution. Picture: iStock

Energy performance is making a deep and lasting mark on the housing market. 

For many homeowners with a moderate spend in hand, there may seem overbearing challenges jumping on board with renewable focused improvements to their energy lode.

Still, together with near passive house energy performance as standard for new builds, there are not only retro-fit grants from the Sustainable Energy Authority of Ireland (SEAI), but useful financial mechanisms on offer to help Irish property owners to increase their homes’ efficiency specs and day to day comfort. 

Domestic households are still driving 30% of our greenhouse gas emissions.

Here, we’ll take a look at one of the proven, SEAI=supported, sustainable energy retrofit projects — a pumped, solar thermal water heating system. There are a number of more advanced solar hot water systems and this one is the most basic and familiar.

If you already have PV solar panels making electricity for your home, a dedicated water diverter can do the work of solar thermal, contributing to your everyday water needs using solar energy directly (not drawing from your battery). The Eddie diverter for wind turbines and grid-tied PV is a familiar name here in Ireland.
If you already have PV solar panels making electricity for your home, a dedicated water diverter can do the work of solar thermal, contributing to your everyday water needs using solar energy directly (not drawing from your battery). The Eddie diverter for wind turbines and grid-tied PV is a familiar name here in Ireland.

Since the introduction of grant aid for more pricey but versatile photovoltaic systems in 2018 (solar PV), figures from the SEAI show a massive increase in the uptake of solar PV grant aid, and a similar plummet in the demand for solar thermal projects. There were just 293 SEAI grant-aided solar thermal systems installed last year, compared to 2583 solar PV systems. Even with the interruption of COVID 19 — that’s startling.

We remain somewhat nervy about the usefulness of solar energy in Ireland. The truth is, it’s not the blazing sunshine that we’re after — it’s light, solar iridescence, which here is as much as 60% diffused sunlight, reaching us even through cloud. 

I can vouch for my solar PV system as providing 90% of all my electricity from April to early October, and 50% plus the rest of the year utilising a 6.5kWh of battery storage.

If you choose solar PV, it’s possible to have a micro-generator/water-diverter working directly from your PV panel power, contributing free solar electricity towards heating your tanked, hot water supply. This is ideal for sink washing, mixer showers etc. The Eddie is a popular brand here in Ireland, occasionally offered as a sweetener with a full PV/battery system.

Solar thermal still offers a cheaper, effective retrofit to bring your existing water supply (the domestic supply and a drip of your wet central heating supply for low temp UFH) up to a higher temperature.

There are two types of solar thermal collectors, sealed vacuum tubes set on-roof, or highly insulated flat panels. Flat-panel collectors have a collector plate set over welded on copper tubing under solar glass, in a sleek, insulated neat frame. They can be set flush into the roof or mounted on a frame. Like PV panels, they are arranged orientated towards the sun on the roof of the house or sometimes on nearby outbuildings.

A solar thermal system has to be plumbed in, and works on a loop of small bore pipework that runs from your roof panels or tubes through your water storage tank. The cycle is run by a controller which responds to the degree of available energy being gathered. There’s two coils going through your cylinder (we need a twin coil cylinder), the second, being the supplementary solar thermal system.

The heat transfer liquid (usually a blend of glycol and water) is pumped mechanically up to the roof with a little electricity, passes through the panels or tubes where it picks up that free, renewable solar energy, and returns thermal energy to your storage tank through a heat-exchanger generally housed in the tank. Your conventional boiler or immersion takes over to bring the supply to a safe, consistent temperature. If you don’t have a cylinder – conventional solar thermal water heating and solar PV diverters are not viable.

If you have an un-shaded roof with a pitch of 20 degrees to  45 degrees (that’s the ideal), enough room for solar panels, facing full south or up to 40 degrees east or west of full south, you have a choice to make. Though claims vary between supplier to supplier, you should in most cases enjoy 50%-60% of VAT free savings on the cost of your hot water using a well detailed solar thermal array.

Solar thermal systems have more moving parts than PV, require significant plumbing and focus solely on the hot water supply. Although less sensitive than solar PV panels to cloud cover, it would take a vast array of solar thermal to contribute meaningfully to wet central heating even using low flow UFH. The SEAI estimate 8m2 of panels would deliver just 2% savings on CH costs for an average detached house. Manage expectations, and don’t be blathered into disappointment by a less reputable supplier.

Our hot water experiences are a bit anecdotal, as we use electric showers but on any bright day in the warmer months, the immersion tank is full of steaming hot water.

In terms of aesthetics solar thermal plates are easier on the eye, similar to PV plates but slightly smaller. Vacuum tubes have a rude grill formation. 

Their impact will depend on the pitch, height and aspect of your roof. Personally, after the initial “yikes” I’ve found that roof mounted plates become less noticeable over time, especially when bragging rights concerning money-saving kicks in. 

The advantages of staying with evacuated tubes? They are a lot lighter to handle, are more efficient than a plate (so need less square metres) and can be replaced individually if damaged.

In terms of planning permission (PP) for PV or solar thermal arrays, the SEAI advises "the total collector area must not exceed 12sq m or 50% of the home's total roof area, otherwise planning permission will be required; discuss this requirement with your registered contractor”. 

If your roof is not facing full south or within a reasonable SE/SW direction, the size of the array may have to increase for good solar gain,demanding PP and pushing up your outlay.

The cost of a traditional solar thermal system will run (including all system parts and labour) around €800 – €1,300 per square metre of roof panel. You will require around 1m2 – 1.5m2 per person. Alternatively, solar PV is around €1,700-€2,500 per kWp. Despite the difference in capital outlay, you should examine the merits of one system against the other, as punitive carbon taxes are dragging up the price of electricity by the week.

Your supplier should be able to sit down following an on-site survey, and work out a projected payback period based on the capital cost to you (after any grant aid) and your current expense to power your home (if considering PV) and to heat your hot water. This should be easily understood, recorded in writing and a help to you in deciding if this installation is cost-optimal. The savings with solar thermal or the use of a PV system with a hot water diverter, will be noticed more obviously if you’re using LPG, electricity or oil to heat your water right now.

The solar-thermal grant amount allowed by the SEAI is directed at homes built and occupied before 2011, and taps out at €1,200, around 35% of the cost of a standard solar thermal installation. Solar PV grant aid reflects its higher capital cost, capped at €3,000 for 4kWs of panel with a battery (diverters are not grant-aided). If you buy into three or four different, roughly simultaneous SEAI grants, the total grant figure awarded can be poked up by €300 or €400.

Any solar thermal system installed should cover the occupancy expected to live in your house (not the number of people currently present). It’s the size of the house that sets this number. Selecting a solar thermal system that is smaller than the appropriate size set by the square metres of the house; the SEAI grant will be scaled down. Obviously, diving into the plumbing, it’s vital that the entire system is correct. Once you get the SEAI grant approved, you have 8 months to get your solar thermal system in place.

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