Tilting at the Sun - Vertical Solar Panels

Do Vertical Solar Panels get More Energy in Winter?

Can you install solar heating panels to collect more energy in winter?  If you tip them up vertically and over-size them, is it possible to get nearly constant heat output throughout the year?

Works in Theory?

In wintertime the sun follows a lower path across the sky, in summer the sun rises higher. The concept is straightforward – by installing solar panels at a vertical angle they face more directly towards the winter sun and should collect more energy at that time of year.  Of course, vertically installed panels also collect less heat in the summer, but since solar thermal systems often produce more energy than can be used in the building at that time of year, perhaps you could get more coverage of your hot water with vertical panels.

We set up a model in a solar thermal modeling software (T*Sol), and simulated a year of hot water generation with solar panels set up at a range of different tilt angles.  We did this for a “right size” system, 6m² square metres of flat plate solar thermal panels serving a hot water use of 200 litres per day at 50C, and an “over size” system of 18m² serving the same demand.

The table below shows the prediction of the simulation software for this range of systems.  The orange colour is the weekly energy in hot water used, and the yellow colour is the energy supplied by the solar thermal system.

Weekly Solar Energy (yellow) for Cambridge, UK 200litres/day water use (orange)

(Thanks to Ian Barclay at Viridian Solar for the modelling)

To make things a little clearer, here is a graph contrasting the solar energy delivered by panels installed at 35° and panels installed at 75° for the 6m² system.  The more vertical panel produces much less energy in summer and only a little more in winter.  The loss in summer is not compensated by the gain in winter.

The reduction of summer energy is not compensated by increase in winter

Perhaps if we oversize the system to produce a massive excess in summer, can we bring up the winter energy and get a more constant output through the year?  The graph below does the same comparison for the 18m² system. 

Even for oversized system, winter gains are less than summer loss

The larger system produces near 100% hot water coverage for much more of the year than the smaller system, but the effect of tilting towards vertical is the same as for the smaller system – the extra energy in winter does not compensate for that lost in summer.  Note also the effect of diminishing returns in solar thermal design – we have tripled the solar panel area for a 50% increase in solar energy (read more on this effect here).

Perhaps we should blame the UK climate?  What if we were to consider a more continental climate.  Cold, but sunny winters instead of the mild, cloudy, gloomy UK weather?  We repeated the analysis with climate data from Dresden, a location with a continental climate having a latitude similar to Cambridge. 

For Dresden, Germany the results are similar to the UK

The results show a similar pattern to the UK simulation.

Our modelling suggests that the main effect of tilting solar panels towards vertical is to reduce the summer energy collection.  Although the sun is lower in winter, it’s also weaker and more often obscured by clouds, so the winter benefits are small.

So much for theoretical modeling, does anyone have any real-word experience to share?

Do Heat Pumps Deliver?

Do heat pumps actually deliver the energy and carbon savings that housing professionals and energy assessors predict?  A study by the Energy Saving Trust (EST) appears to show that they don't.

The measure of the performance of a heat pump is the coefficient of performance (COP).  This is the ratio of the heat energy that it delivers to the electrical energy that the unit consumes to do so.  An electric fan heater or storage heater would have a coefficient of performance of 1.0 - every unit of electricity consumed in the device ends up as a unit of heat energy in the building.  Heat pumps use electricity more intelligently than this, driving a condensation cycle which cools the ground or air outside a building and delivers the energy as higher temperature heat inside.  The technology is similar to a refrigerator, which transfers heat from inside the fridge to the room outside using the same principle.

For a heat pump to produce a carbon saving relative to a gas-fired boiler in the UK, it needs to be operating at a COP higher than 2.35.  This is because grid electricity has a high carbon-intensity.  When you consume a unit (kWh) of electricity, 0.52kg of carbon dioxide are emitted at a power station.  When you use a gas boiler to produce a unit of heat for your home, around 0.22kg of carbon dioxide are emitted from its flue.

70% of homes would have had lower carbon emissions with a gas boiler

The chart above shows the range of COPs measured in the EST trials (white) compared to the range of COPs claimed by manufacturers (orange) and used to calculate household energy for building regulations.

Fully 70% of households in the EST field trial would have lower carbon emissions from heating their properties if they had instead selected much cheaper gas-boilers.  It also appears that building regulations calculations are over-stating the efficiency of heat pumps by around 60% compared to the real-life examples in the trial.

Can any professionals from heat pump companies offer an explanation for the EST trial results, or highlight errors in this analysis?  Please leave your comments below.  Check workings here if you like.

Note Added 28/03/2012:

Thanks for all the comments to the original post.  The explanation appear to be that  heat pump performance is strongly diminished if it is:

• attached to inappropriately sized heat emitters (for example too small radiators)
• sized wrongly for the heat load
• not used correctly (for example run hard for short periods)

Following on from the EST trial, a group of trade associations created a document to help with the first two issues.  The heat emitter guide helps installers of heat pumps ensure that they achieve more optimal system design and achieve heat pump performance that approaches the laboratory test conditions.