Lithium Ion Capacitors (LICs) are a great alternative to traditional lithium ion batteries in many LPWAN applications. This LIC module which comes with a 250F LIC and a 3.3V regulated output.

There's also a newer/better version of this harvesting module which uses AEM10941 from e-peas.

Solar powered version has been running for more than 6 months now (over the winter) without interruption. Here's the Datacake IoT dashboard for it.