I strongly recommend the following three journal articles describing a highly-innovative design in which concentrated photovoltaic thermal (CPVT) solar collectors are integrated into a vegetation greenhouse:

1) Sonneveld PJ, Swinkels GLAM, Bot GPA, Flamand G. Feasibility study for combining cooling and high grade energy production in a solar greenhouse. Biosystems Engineering 2010;105(1):51–8.

2) Sonneveld PJ, Swinkels GLAM, Campen J, van Tuijl BAJ, Janssen HJJ, Bot GPA. Performance results of a solar greenhouse combining electrical and thermal energy production. Biosystems Engineering 2010;106(1):48–57.

3) Sonneveld PJ, Swinkels GLAM, van Tuijl BAJ, Janssen HJJ, Campen J, Bot GPA. Performance of a concentrated photovoltaic energy system with static linear Fresnel lenses. Solar Energy 2011;85(3):432–42.

It will always be most efficient to use sunlight directly for the plants. This is because typical solar cells and LED lights are both around (usually less than) 25% efficient, so you are only getting ~6% direct energy transfer from the sun to the plants. 

You could, however, use PV, batteries, and LEDs to extend the effective length of the day. This would be necessary in the winter to trick plants into flowering/fruiting, etc.

Thanks Russell,

The issue is that when PV modules are placed on top of the greenhouse it creates shadows inside it and this causes parts of the greenhouse's vegetation to produce less (due to shading). My question is whether anyone has information on what vegetation would NOT be so much affected by this phenomena. I know that palm trees are not affected, but we are interested to grow vegetables etc... rather than palm trees

Your question cannot have a simple answer, because it involves many issues. The first one is: why do you want to put a PV plant on it? If you want to heat the greenhouse, you have to do a technical-financial comparison among a PV plant and other solar technologies. Secondly, you can put the PV modules onto the central area of the greenhouse, such that the shading is limited to few hours/day. thirdly, you can use semi-transparent PV modules, to reduce the shading area. Fourthly, you have to discuss on a specific object which will grow into the greenhouse, because the PAR could be very different...In an our study, we've verified that the roses can grow into a PV greenhouse, if the PV module cover only the central area.

Thanks SIlvano,

The idea is that when you have very little space to generate electricity, you start thinking of different options. One of these options is to use the greenhouses as a supporting structure for PV without affecting the growth rate of plants. Hence, there has to be limited shading or new designs are made to avoid shading altogether.

In this case, you should think about a solution with semi-transparent PV modules, iintegrated into the structure of the greenhouse.

Indeed but I was asking if anyone has had experience with such an installation because the semi-transparent panels will still absorb the UV and visible light, which is also needed for photosynthesis.

By changing the design of house you can achieve the Goal, Such that keeping the roof and front wall of Green house transparent while put PV on the remaining walls directed towards the Sun. Or by designing such green houses which tracks sun moves in the direction of sun to get the maximum irradiance.

Thank you Muhammad. Unfortunately, your suggestion only applies for new greenhouses. Our greenhouse is already existing and we wanted to know if anyone has had the experience of retrofitting PV modules to existing greenhouses and how would that have affected the produce of the crops.