Hi,
i just did some experiments here with ultrasound distance sensor in a pvc pipe and I got the opposite behaviour, the range decrease 
So could you share a bit more information about size of the PVC pipe, diameter? Thickness of the PVC?
Or maybe it is due to the sensor, I use the HC-SR04 which I bought from dealextreme - Recommendations For You - DealeXtreme
HY-SRF05 seems to be some kind of improvement of mine and not much more expensive. Anyone who has any experience on both of these?
Regarding solar powering I have some experience from a similar setup. I have built a number of wind stations(http://www.blast.nu/products) that measure both wind speed and direction and report these reading to a webservice. To power these I have tested a couple of different solarpanel solutions. It is very dependent on where you going to place the units. Here in Åre(northern Sweden) the 50W solarpanel and a 6.5 Ah lead battery is basically sufficient to power my units up to november. After that the sun is too low on the horizon so the battery wont charge. In february/march the sun is strong enough again here. So last winter we used instead old car batteries 50-60Ah which we had to manually replace every fourth week.
What you need and what is sufficient is of course very depending on how much power you draw and it is not very easy to calculate unfortunately. From experiments I have concluded my wind stations draw about 100 mA. The arduino is constantly on, the GSM unit is turned on every 5th minute and transmits the readings for the last 5 minutes and then shuts down.
/Kalle