What apps to use if I want to turn my old phone into a bicycle computer? I want to view basic info all the time such as speed, trip time, total distance traveled etc.
Are there any apps which will save energy while showing these details?
This is possible.
Firstly, it depends on your idea of "old" For some users thats any phone over five minutes is old, and for others old means a flip phone from the 90s.
Assuming you have an android-based or ios-based phone that works and has a decent battery life then you're pretty much set. Both will run the Strava application, which looks something like this:
As @dancek says, the home screen is not configurable, so you will only see average speed for the whole ride, and instant heart rate/cadence rate if you have the sensors. Time and Distance are counters
This is the screen that shows up when you stop. The map with track at the top can be displayed full-screen while riding, and if you have planned a route the track is overlaid on the map as well.
Later analysis can be done on the info - here's the link for that ride: https://www.strava.com/activities/681005599/analysis Note I had a HRM but no cadence sensor.
This is the RUN view if you're into that kinda sport.
If you have an applewatch there's a thing for that, but I have no experience of this. It looks like this:
Both of those screenshots show heartrate. A phone can't do that alone - you need a heartrate sensor. Phones with ANT+ radios are better than bluetooth sensors because the batteries last for 4-6 months, whereas bluetooth sensors are a much shorter runtime. Also note, that phones with a "pulse meter" are useless for this purpose.
There are ant+ cadence sensors too - often a little box that straps onto a chainstay, and a small magnet on the end of a crankarm that passes near the sensor.
If you want to see your phone while riding that gets messier. Personally I keep my phone in my pocket with the screen off.
Otherwise you'll need a handlebar mount, or a top tube bag, or both. Its a good idea to bring a USB battery pack and cable for your phone, because a phone that runs for a day of normal use might only get 2-4 hours of riding time before its flat.
Right-angle USB cables can help organise things too.
Windows phones are lacking in this area - a good answer is Windows Phone as a Bike Computer - What is the current State of the Art?
A quick Google search of "Andriod cycling apps" will give a a thousand apps, 500 reviews and 100 "best for cycling" lists. Anything in the top 20 of any of the lists will do that is being asked.
So the real question is which ones "save energy" - I presume you mean use the least amount of battery power. The problem with this approach is really down to relying on GPS, and the nature of cell phone screens, which both large amounts of power. A normal phone can get 4-8hours life with GPS on, and the back light needed to run the display reduces it to a few hours at most.
The amount of power the application uses is insignificant compared to the above two, so to all practical purposes, any app that meets your requirements will use the same amount of power. Unfortunately, because of these battery life limitations amoung other things, cell phones are not idea bike computers, which is why there is still a huge market for dedicated bike computers.
If you want to get the cell phone to last as long as possible, you need an app hat will turn the screen of and leave the screen off unless you explicitly look at it. If you really want real time data you can see at any time without pressing buttons, a $20 wired cycle computer is still the most reliable and cheapest way to get it.
As far as applications, I currently us iCardio on my andriod phone and find it does everything I want and more. I won't recommend it over others, as I haven't used anything else for years.