Can I increase linux entropy by using rngd or script?

I want to continuously increase /prco/sys/random/entropy_avail when it reduced.

I first check the rngd (http://ift.tt/2d2AWGQ)

It says /dev/random is very slow since it only collects entropy from device drivers and other (slow) sources and I think that is why we use rngd.

And it says rngd mainly uses hardware random number generators (TRNG), present in modern hardware like recent AMD/Intel processors, Via Nano or even Raspberry Pi.

problem is here. When I start rngd it says

[root@localhost init.d]# rngd 
can't open entropy source(tpm or intel/amd rng)
Maybe RNG device modules are not loaded

But I don't have intel rng confirmed by cat /proc/cpuinfo | grep rdrand

[root@localhost init.d]# cat /proc/cpuinfo | grep rdrand | wc -l
0

If there is any possible resources that I can use???
or
Is it possible making script to increase /proc/sys/random/entropy_avail ??

Thanks. Any comments are appriciated.