Photovoltaics for Domestic Hot Water

An Eccentric Anomaly: Ed Davies's Blog

I've previously written about the possibility of using the output of photovoltaic panels for heating applications in circumstances where traditionally solar thermal (evacuated tube or flat plate) panels would be used: PV For Space Heating, PV, ETs and Flatties and, slightly less interestingly, Solar By Area.

Since then the price of PV has continued to drop. At the moment Navitron, for example, are doing Kinve 235 W panels at a fraction over £0.57/W (incl. VAT) compared with another supplier's £0.74/W + VAT at the beginning of the year. In the mean time I've been fiddling around doing a calculator web page to try to explore the trade offs in systems which combine solar thermal with a heat pump driven by photovoltaic panels. I'm not happy with the design of this yet (it works as intended but it's difficult to get anything very insightful out of it) but playing with it a bit did show over how wide a range of operating conditions the costs of output power of PV and solar thermal are similar.

This page is, therefore, a simplified version of that calculator designed specifically for comparing the cost of power from PV and solar thermal.

You need to specify all of the parameters defining the components and operating conditions of the system except that you specify a range for one parameter (which will be used as the x axis of the result graph) and a list of values for zero or one other parameter which will result in separate curves in the result graph for each value in the list.

Specify a range by separating two values with a hyphen-minus (e.g., to plot for insolations in the range 100 to 400 W/m² use '100-400') or by leaving the field blank for a default range.

Separate a list of values using semi-colons. E.g., to plot for insolations of 100, 200, 600 and 1000 W/m² use '100;200;600;1000'.

The results are cut off when insolation or other conditions mean that the cost of output of a panel exceeds £6.00/W (by default, set other values using the Cco variable). This is mostly just to keep the values on the Y-axis spread out for readability but it makes physical sense too: at low output things are sufficiently marginal that you don't want to be relying on operation anyway.

More on the use, caveats and interpretation of this calculator in later blog posts....

Solar Thermal Panel Characteristics

Ast Area 0-5
η0 Zero-temperature-difference efficiency Dimensionless fraction 0-1
α1 Heat loss proportional to temperature difference W/(m²·K) 0-2
α2 Heat loss proportional to square of temperature difference W/(m²·K²) 0-0.01
Cst Cost of panel pennies 0-100000


Photovoltaic Panel Characteristics

Pnom Output power under standard test conditions (1000 W/m² insolation, etc) W 0-500
G0pv Insolation at which panel output drops to zero W/m² 0-100
Cpv Cost of panel pennies 0-50000


Operating Conditions

G Insolation W/m² 100-1000
Tout Temperature of the output (above ambient) °C 0-100
Cco Cut-off cost, cost of power above which we don't plot pennies/watt