Daily Archives: April 15, 2009

Solar Panels Calculations

At last I have found some reasonably priced solar panels; basically I have been researching suppliers for 30 tube (vacuum tube) 58MM diameter and around 1800mm long solar panels. Prices have ranged from extremely efficient (95%) 30 tube panel sets from around £2K per panel; to 80% efficient 30 tube panels for £400 and less (to BS standard BS EN12975-2 ). So very much a no brainer with regard to which panel to choose by the time you have factored in the cost of additional damage prevention systems for the cheaper panels there is still a considerable cost saving.

Solar Panels at the Crown the costs and the benefits

We have 60m of available width space to fit solar panels to the roof, this means we can reasonably fit 27 panels maximum to the roof, though it will be around 25 due to gaps between panels

The total costs of these panels at 30 vacuum tubes each will be around £400 each (est. present costs are £433 before discounts) panel, I have allowed for £100 of fittings and tubing per panel in the costing
and an equal amount of labour.
The panels come with a fixing kit for mounting on a flat roof do have a look at http://www.top50solar.org.uk/ for some suppliers, panels can be supplied with the flat roof stand or pitched roof mounts.
The above suppliers are the most competitive suppliers I have found from 50 suppliers and I have narrowed down prospective quotes for 30 tube solar vacuum panels to go to 6 suppliers.

Assumptions

  • local solar values have been used,
  • 80% efficiency of the vacuum solar tubes (flat panels are roughly 75% efficient), (there are two types of solar panel flat and tubular)
  • tilt angle is latitude + and – 15 degrees, though more probable that only the positive tilt will be used from 54 degree angle (so 39 to 70 degree tilt at 54 latitude)
  • direction of facing panels is SE and SW (SW is slightly more optimal for winter performance)
  • days sunshine data and KW hr’s per M2 (from last 60 years at High Mowpeth),
  • total heat store of 2.5K litres with 90 KW coils per cylinder (4 cylinders in total),
  • a suspected doubling of gas costs in the next 3 years from 3.5 per KW hr (inc.ccl) to 7.5p per KW Hr,
  • sunk (past) costs up to today’s date are ignored, (these total £11k at today’s PV values after tax)
  • maintenance costs of £200 per year after year 4 (I presume we will have to replace faulty tubes etc..)
  • cost of capital of 10%
  • max number of 30 tube panels fitted is 25 (though might be able to get 27)
  • each 30 tube panel is roughly 2M long (hypotenuse), 1.6M max vertical height and 2.5M wide and 100KG full weight (if an un-mounted solution is used then the height will vary between 1.6M and 1.2M.

Break Even

The total cost of the 25 30 tube panels should be £12,500 (was 75K 4 years ago!)

The total cost of fitting the tubes with extra pipes and kit (inc. using the pool as a thermal dump) £5,000
I was intending helping Paul (our heating engineer) do the work on the roof as a change is as good as a holiday.

Break even for the project is in 4 years
Break Even Of Project

If I include past costs this is extended to 7.5 years.

How much hot water can we produce?

% Excess Heat

From early April to mid August excess heat will be produced, the excess quantity produced is to a degree
difficult to calculate as some excess may be “put back” into the hotel hot water system and not be an excess
i.e. if a period of “heavy draw” is experienced.
However the main thing to realise is that all the excess heat will be recovered and dumped into the pool;
after putting these numbers through some more number crunching taking into account
the day time air temperatures, temperature of the solar fluid reaching the pool heat exchanger etc…
roughly a percentage % of the pool heating bill will be supplied from excess heat from the solar panels as follows:

  • For May 25% of pool heating will be solar,
  • For June 50%,
  • For July 30%,
  • For August 15%,

KWH excess heat Produced

The beauty of using the pool as a heat store is that if there is very little hot water draw on any given day the excess heat will
Be dumped automatically into the pool. This is very possible in March, April and September and October months.

I have a table below based on local historic data for sunlight and kwh’s per M2 sunlight for the Scarborough latitude

Jan Feb Mar Apl May Jun July Aug Sept Oct Nov Dec
Days in month 31 28 31 30 31 30 31 31 30 31 30 31
Kw hr Per M2 Per Day average 0.64 1.17 1.94 3.32 4.17 5.00 4.44 3.61 2.78 1.67 0.78 0.47
Average Sunlight per Day in Scarborough area last 60 year average 1.6 2.4 3.2 4.5 6.0 5.8 5.8 5.6 4.4 3.3 2.1 1.3

The direction of a facing panel also alters the efficiency in northern latitudes basically south is not always the most optimal in the real worlds data though somewhere between SE and SW should be fine.

Tilt can also alter efficiency this is generally the degree of latitude you are at (54 degrees at Scarborough) though in summer a panel will give generally more than enough heat so maximising the winter sun is often preferred in northern latitudes (as seen in table above). To maximise summer heat have a tilt at your latitude +15 and to maximise in winter (and reduce the chance of your panel blowing away) tilt at latitude -15. So basically adjust your tilt twice a year if you can, tilt up in Spring and down in winter.

Basically you can now work out the theoretical average energy per month from a solar panel (very easy with a spreadsheet)

Flat panels work to roughly 75% but can often be a lot less at northern climes (even if you tilt adjust), a vacuum tube works at around 80% efficiency if correctly facing roughly SE to SW and tilt adjusted.

If you know your absorption area (e.g… 2.4m2)

You can now calculate the energy produced per panel so a 2.4m2 panel in vacuum tube (80 % efficient) format will produce a maximum of about 9.6KWh’s per day in June (5.0*2.4*0.8), about enough to heat all the hot water for a 3 bed house plus some extra. However do look at the winter period where this is reduced to around 1KWh per day. Therefore, you have to choose between paying for extra panels or to rely on other mean of heating hot water during the winter period; but do remember these are MAX values! So in real life result will vary from site to site.